Magress' Sphere, Black Lance:
30% HP and DEF, 5 BC regen on basic attack and 20% drain attack.

So I'm thinking that the new GGC Black Lance sphere would be best used for mostly self healing / def purposes while being offensive.

My reasoning:

While the 5 BC off normal attack makes it a somewhat viable sphere for arena, sol creator and for sure, halloween spheres and lexida trump it in use for raw bc generation for the team.
The defence and hp boost from it can help you survive from retaliation from turn 1 but lexida does provide the hp portion.

So perhaps the sphere is good in questing?

Again, I don't quite think so, since generally you'll want to go offensive mode in questing and steamroll everything.

Then for boss battles?

Here I feel, the Black Lance is perhaps useful, over some other spheres.
And to better explain my thinking, I'll use Maxwell from Trials 3 as an example: Using a typical team where lilith / rowgen are the main attacking force against the boss, i.e, you only have 1 or 2 attackers doing worthwhile DPS, and everyone else is generally guarding, sometimes throwing off a heal or damage mitigation, the attackers are more susceptible (in general) to eating damage.

But with infinite SBB use, the 5 BC generation part of the sphere is lost. The healing portion however will be constant and potent . Providing perhaps full heals each turn for your unit, rewarding them for being offensive.
This is where Black Lance shines i believe. Not the 5 BC, but the HP, Def and constant HP regen.

Now remember, against Maxwell, it's not about BLITZING damage against her, its about sustaining and slowly grinding her down (in general, you might argue that by doing vastly more damage you'd therefore take a shorter time to kill her, and therefore sustain less damage and therefore "a good offence is the best defence").

Now don't worry, I've seen the counter-argument of - "well rowgen and lilith are used OFFENSIVELY so we should shove on purely OFFENSIVE spheres on them such as bathootha with a spark sphere, maximise teh damage!" and I agree wholeheartedly that they are a force to be reckoned with when you pile on the attack and spark with them.
However! Just think about fighting Maxwell, you are only attacking with 1 or 2 units a turn, there's no cloud of other attacks to ensure a large guarantee to hit that spark with rowgen or lilith, the fight centres around survival.

I believe the sphere actually suits rowgen and lilith (and sergio) great because, the 5 BC is actually utilised probably the best with them! You are normal attacking with rowgen or lilith for a chunky amount of time, since you aren't saving up for merely BB, but SBB. You are using a tonne of only normal attacks until you hit that magic infinite SBB moment. The 5 BC actually BOOSTS that unit along a lot and is actually better than evil shard (though they're different type spheres) and provides utility to those units later on when BC isn't an issue anymore.

You can argue that Dandelga would be therefore more worthy, but seeing as its such a perfect fit for Darvanshel or any other Mitigation unit, it's already being used.

To those that say a mitigator could benefit from the Black Lance, I'd say, "maybe" because Dandelga is so potent for Darvanshel since the Black Lance only benefits the unit off normal attack, and Darvanshel / other mitigators are generally defending or BBing or (rarely) SBBing. The HP/Def/Self Heal portion is great however your mitigator is generally very tanky, when it isn't defending, its got its BB/SBB up.

To those saying Lily/ grah will benefit from Black Lance, I'd say...wha? Lily should generally never be attacking, she's the queen of laziness, sitting upon her throne throwing off BBs and SBBS. She isn't an ideal attacker, the HP regen is wasted, and she's defending the rest of the time so HP/DEF isn't really needed either. I'd say that Providence ring with Lily is a great combination, keeping to the theme of benefit from being hit.
Grah...I dunno really, sure he might be attacking normally and then BBing, so might be ok, i guess? He probably enjoys a Legwand more though?

So yeah.

TLDR : my opinion: Black Lance good for units that attack normally loads just to get to SBB of usefulness, with reasonably high DPS for Boss Fights where it's a battle of attrition rather than blitz. No good for OTKO teams, little use for arena, little use for questing (when you have lots of other good spheres). Give Black Lance to the lone attacker, the Black Ranger who carries the team's DPS and he will therefore never die.

Edit: I totally can see the case for shoving it onto units with high SBB fill requirement, only downside is the lack of consistent high DPS and therefore delicious HP regen, but still, pretty good use for the sphere! But when that SBB hits, its like full heal time rofl

Edit 2: Still no consensus yet, this is a fun discussion, reading your guys' ideas, also -

An interesting take on the sphere's use by /u/Adolus is to use it on a healer. At first sounding weird and slightly counter-intuitive, it has some clever parts to it.

For it, I'll use an example actually- for a typical healer, let's take good ol' altri, pretty good / ideal sphere (double) set up would be : Drevas Evil Shard

However with a set up like Dark Lance Refined Gem.... You would receive the HP/Def buffs, No statuses = Drevas , and then its a show down between Evil shard vs 5 BC from attacking. Here it depends on how risky you are with your healer, if you attack with your healer, you can benefit from 5BC your drop checks(bc hc), however on tough battles / stuff you wanna guard a lot against, defending 4BC off E.S would seem better.

But the healer probably won't take full advantage of the heal off D.Lance though, being generally not very powerful attack-wise.

Edit 3 : Guys 20% heal is kinda awesome. Testing Black Lance on Lilith in Magress final stage and she's healing 3-6k hp every turn. Imagine rowgen guys. Full heals every turn anybody?