Expressive, thick and very playable sounds (not too gimmicky or trendy - which is what Reaktor suffers from IMO). Tassman is for people who like to play instruments with expression - whereas Reaktor seems more geared toward instant gratification and one-finger players.

Great features - but I do aknowledge there is always room for improvement.

Everything

Get the modeling bundle and get all of the AAS products for a great price (you can actually get the whole bundle for not much more than the price of Reaktor alone - making it an awesome deal).

Response is great.

Sound quality is better than almost all other software synths. It really can sound like a true instrument.