Atma's is certainly a niche item. however i think it works best on bruisers (surprise surprise)
it allows you to build tanky without sacrificing too much AD. a lot of bruisers can do ridiculous amounts of damage with only 200AD. with a frozen mallet, warmogs, Atmas, BV, and boots with furror (sp?) you can stick to any target and have the stats to survive pretty much anything they throw at you.
As an example:
Darius with warmogs, Frozen mallet, Atmas, BV, and ninja tabi. (last item is open, probably a last whisper or black cleaver would be best, but add stats as you please for the last item)
stats are 4200 health. 88hp/5. 153Ar and 103MR.
we'll have 206 base damage. base DPS would be 233.
burst from abilities over 5 seconds will be 2875.6.
with auto attacks he's doing 1165. for a total of 4040 damage. not to mention a good portion of this is true damage.
Let's compare this to blanc at the same price-point. (11860 gold)
Sorc boots, Deathcap, RoA, Rylai, Blasting wand, Tome.
She will have 2870 health. 83 DPS and 425 AP. so her offensive stat is over twice what darius has.
To make things more even i'm going to assume blanc has blue buff and enough cool down to get sigil off twice.
From abilities she'll have 3627.5 + 415 (AAs) damage. and this is assuming she'll get all her AAs off. unlike darius who will be AAing, leblanc may not constantly AAing. so we're really doing what we can to put this in the favor of blanc.
Her total burst comes to 4042.5
Now I do not consider Darius an assassin. by any means. he's a bruiser, and an executioner. however with his damage he's really more of a tank-assassin hybrid. which, IMO, is blatantly broken.
LeBlanc, i do consider a bruiser. so really at all stages of the game I feel like she should feel extremely powerful in a short time frame. 5 seconds is pretty short. and she can do considerable damage.
but when you look at bruisers, and the fact that they can pull off the same amount of damage, and be twice as tanky...
so no. I have to say that Atmas doesn't need any adjustments right now. if anything i'd nerf it. but it's really not needed. I feel the item is fine the way it is.
bruisers/AD really DO NOT need any better build options. they are fine the way the are and extremely versatile.
We could use a lot more build options for AP right now however.