Agüero isn't a top-class finisher yet (and it isn't his best ability, I should know after watching every single game he's played since he arrived to Europe) but he's improved this aspect of his game a lot over the past year only. He can get you at least 15-20 goals a season and now with better players around him, he'll be able to produce even more, in terms of all productivity. The problem during his first couple of years at Atlético was that he ran out of steam when he had done all the hard work and simply couldn't finish off his chances as the concentration level wasn't high enough in those situations, however, he clearly showed in the past season that he's improved in terms of finishing. He'll also bring a lot in terms of creativity as he's able to put the finall ball to his teammates, dribble past one, two or even three players to set up a goal or finish himself, plus he's a hard worker and always wants to improve. His low centre of gravity is what I love most about his game; he has got a terrifick balance and can push away much bigger players than him in those type of situations. The amount of times he's turned a player inside out is just remarkable and he also shoots with both his feet, the right being better though.
The only thing that's been a problem is all the muscle injuries he's suffered. He only completed like 66% of all training sessions from August-January last season and then improved his physical status, so that's the only thing I'd be worrying about.
Fantastic player who's done a lot for Atlético, a shame he had to act like a complete bastard during these two months though. Btw, he's a true professional, so don't expect him to "love" your club.