He's got huge talent but a bit rough around the edges, pretty much as you could say about Bale before this season. He doesn't look like a £20mill players at the moment but if things click for him in England that may change. I guess that makes him a bit like Oscar, he's not anywhere near worth that but given your funds you can take a gamble on how good he could be in 2/3 years time.
As for his style he's a direct player, can run at players with the ball or knock it past them but his end product is erratic, his crossing isn't great but he seems to spend most of his time cutting inside anyway and his shots can go anywhere, at times he'll finish clinically but too many times he'll mess up really good chances and from outside the box it's the same story, he wastes a lot of the ball but every now and then one goes flying in. The Bundesliege suits him as it's very easy to counterattack, there are very few sides that will actually set up to defend from the off, infact Leverkusen are one of the few that defend well(no surprise with Hyypia in charge) which is probably why they're so high up the league. I guess it depends how you end up playing next season, if you get Mourinho and he has his way, I think he'll suit your team but if Abramovich still wants to see Chelsea play like Barcelona, he won't. Personally I think he's best playing off the striker, rather than pretending to be a winger and cutting inside all the time and if he improves his finishing then ultimately I think he'll make a great striker.
Sorry for going on, but Leverkusen are the team I follow most in Germany. You could do a lot worse than signing Reinartz too whilst you're talking to them, he's like Mikel except he's not error prone and reads the game very well.