I finished my first playthrough as Alchemist, and I think it is an excellent spec.
The talent that increases bonuses and drastically decreases the side-effects of potions is absolutely excellent, and will be part of all my builds.
Otherwise, I think the later talents that boost you when poisoned are also excellent, including the capstone one that boosts adrenaline generation by 50% and damage and protection by 15%.
In particular, I loved how the Vigour regeneration bonuses worked. I basically had +50% vigour regen when poisoned, and when coupled with the +40% from swordmanship it meant I never needed to boost my vigour regen with the potions themselves. This means I was free to focus on other effects.
So, for example, I could get Thunderbolt and Rook and Swallow. The potions would give me some health regen (more than offsetting Thunderbold), and +49% damage. Then my "poisoned" buffs would give me an ADDITIONAL +30% damage, +15% protection, AND +50% vigour regen.
The end result is the mere act of taking a potion turned me in a killing machine.
That said, I do not think you should only use Alchemy. Mix it up with some Swordmanship (for risposte and twirl, and the vigour regen), I also went with maxed out Aard. Ended up with a monstruous build. :)
Now, it IS true that in a couple of instance (yes, just 2) you will be forced to fight without potions. It does suck. Namely it is the mist battles at the end of Act 2, and POTENTIALLY the final fight if you spend too much time chatting (which really, you should, the plot info is worth it).
But overall, I think the alchemist build is the most fun, as you need to utilize this most unique and original feature of The Witcher games regularly in order to benefit, and it really is the most fun, IMO.
Thank you.
Itkovian