Being EA and a time to experiment, I created a party of four clerics (one for each diety). I wanted to see how the various Domains compared to each other. The Domains that I choose were: Sun, Battle, Oblivion and Insight. Here's my impression of the domains and how useful they were. I've included a few suggestions on how to improve the under-performing ones.
The character I created here was a Dwarf, so he was able to wield a Warhammer. The special of enemies having disadvantage on Sacred Flames is huge. The spell is almost useless without this ability. With the disadvantage on enemy saves, it becomes a viable ranged option since it ignores cover and light levels that make crossbow useless in many occasions.
The channel divinity is okay, although I totally forgot to use it most of the time. Spell selection is also okay. Fairly balanced and a decent subclass. Depending on your stats, it can make a decent front-line combatant. (As a dwarf, the character had just as much health as most fighters. The only thing lacking is the ability to wear heavy armor.)
The level 6 abilities sound very useful, alas, I never got the chance to use them.
This domain is pretty much your Swiss Army knife character.
You want offence? How about Martial weapons and Magic Missile at first level. Then you get the very fun Flaming Sphere at 3rd and finally Fireball at 5th.
You want defense? You have healing spells and Shield of Faith. Oh, and you can grant yourself temporary hit points once a day too.
The Decisive Strike channel divinity is the channel that I used the most out of all of the clerics in all of the battles, mainly because you're prompted with a "do you want to use it" every time you hit a foe as long as you haven't used it yet. The extra damage is always welcome and the stun can help if the enemy fails their save. No more powerful than Paladin abilities, so well balanced.
Didn't get to try out the level 6 ability. Could be useful, although if the range is only adjacent tiles, might be of limited usefulness.
This domain can easily replace a Fighter and possibly even a Paladin in your party. Very strong.
If you thought the Battle domain was strong, you might want to ditch it for an Oblivion cleric. You get Chill Touch as a bonus cantrip so that you don't have to use the rather pathetic Sacred Flame. (Although, in my case, my Cleric was a high DEX Sylvan Elf so she just used a Longbow and didn't use any cantrips.)
Your channel divinity doesn't do a lot of damage, but it is a Point Blank Area of Effect so you can theoretically hit a ton of monsters at one time and if they fail their save they're all poisoned too. This ability can quickly turn the tide of battle at lower levels with the right positioning. At 6th level you do +d6 Necrotic on your first hit which is almost as good as a second attack. Spell selection is decent, if not quite as good as the Battle domain. Sleep can turn the tide of a low level battle. Bane, followed by Ray of Enfeeblement can take the "ouch" out of a melee boss.
The big plus for Oblivion though, which is possibly over-powered, is the "Peaceful Rest" passive ability. After the very first scripted (so the ability doesn't kick in) night-time attack to introduce the surprise mechanic to players, any time you're attacked at night while travelling it's the bad guys who will be surprised. This is a huge advantage. instead of being helpless pincushions for the first round of battle, you get a free round with Advantage on all attacks. My party usually managed to kill 1 or 2 attackers before they had a chance to react! In one ambush by 3 eagles, I killed them all in the first round. This ability alone makes an Oblivion cleric almost a requirement for a decent party. Who needs the healing of a Life cleric when you can just kill everything before it can even shoot at you???
If you're able to give this cleric the Law Keeper background to pick up Martial weapon proficiency, it can make a viable Fighter replacement. Wade into battle to surround yourself with foes and then pop channel divinity and then slice away with a Greatsword. At level 6, your Greatsword will do 2d6+d6+STR. If you want protection, you can always do Longsword with Shield for +2AC and damage of d8+d6+STR. Or, play a Dwarf or Elf where you get either Warhammer or Longsword proficiency for free. Plus, both races have a subrace with +1 WIS.
I played a Half-Elf for my insight cleric. I gave him the Law Keeper background so that he'd be able to use Martial weapons, which is a good thing because this is one seriously under-powered domain. Sure, the advantage on History, Arcana and Nature sounds nice and it certainly kicked in and helped in the Library, but... I suspect that there wouldn't have been any penalty if I had failed all of those checks, so... useful? Probably not.
Spell selection, compared to any other domain, is basically useless. Sure, I used Identify, but only because I had 4 clerics and no Wizard. In fact, not wanting to have a Wizard in your party and using an Insight cleric as a slightly more robust replacement is really the only time one would want to take this domain. Compared to the others, this is seriously lacking in anything useful.
At the very least, it should get True Strike as a bonus cantrip (in addition to everything it already has). Possibly Guidance for free as well. Both would fit thematically, and since they take Actions to use they're almost detrimental except to buff a Fighter in a boss fight where the boss has a high AC.
The channel divinity to grant enemies disadvantage might be useful, but I never used it. Because the character had martial weapons proficiency, I just gave him a Heavy Crossbow and had him stand in the back while the Sun and Battle clerics played tank.
Life (5/10) - from a different playthrough with a party of Fighter, Ranger (Lowlife, so used in place of Rogue), Cleric (life) and Wizard
The ability to use Heavy Armor is nice, especially if you want to dump DEX. However, if you are playing as a character with a DEX bonus (Elf, Halfling, or even Half-Elf and Human) and get 14 in DEX, Chain mail gets you the same AC as Scale + DEX bonus. So your mileage on this will vary depending on your stats.
Not having Bless and Cure Wounds count against your spell selections is certainly nice because otherwise Cure Wounds (at least) is pretty much mandatory, but the rest of the cleric spells aren't anything so special that it matters. I mean, if you're role is dedicated healer, you're not going to be burning your spells slots on Bane or Detect Magic -- you're going to save them to use with Cure Wounds, right? So... even though it totally fits the domain and I would not change it, I have to rate the spell selection as "lame".
Bonus healing from "Preserve Life" is nice, but with Hit Die usage on a short rest, potions, scrolls and other things like Fighter's Second Wind or Paladin Lay on Hands, it's not really a make-or-break thing. It's also very situational in that the target has to be pretty close to death before it's useful (as it caps the healing at 50% of max health). Similarly, the extra +2 to healing spells is nice, but usually not going to make a huge difference.
This domain is okay if you really want to adhere to the classic Tank/Heal/DPS roles because it just screams "Heal". If you pair it with the Law Keeper background you get martial weapons to go with your heavy armor so until Fighters get extra attack at 5th you're a "Fighter with Benefits". For my taste, a more offence-oriented cleric (Sun or Battle) is better because who needs healing if the enemies are all dead and can't hurt you, right?
Seeing as how this is all straight out of the PHB, I don't see any room to change anything, although compared to some of the other domains, it's a bit lacking.