Well, he had an excellent spell with Valencia 2001-2004. He inherited an average Liverpool team from Houllier, and made it better. Gerrard became a much better player in CM, as did Carra in CB role. Yes he has made transfer flaws, like for example when he hurriedly bought Robbie Keane, and hurriedly sold him. But at the same time, Reina, Mascherano, Alonso, Torres? Not forgetting, he stopped Gerrard going to Chelsea, convincing him to stay at Liverpool.
Then on the negative side, the man management side could have been much better. And Gerrard admitted himself he longed for a "well done" from Rafa. And he was pretty ruthless in his squad selection aswell.
And on the positive, Champions League winners. Yes, Gerrard probably provided the inspiration, but it was still a trophy in the cabinet for Anfield and a massive trophy at that. Then there was the FA Cup in 2006, beating Manchester United AND Chelsea to win it.
Yes, he was slated for being too defensive or negative in his tactic selection sometimes, but at the same time, there was also entertainment, and at times success aswell. He had to balance carefully between attractive, attacking successful football, and plumping for effecient wins when need be. The downfall imo for Rafa at LFC was lack of backing in the transfer market, even though he said Liverpool needed players. I think the board were partly responsible for that scenario.
But I do think he's an excellent manager. Yes recently he's had a bit of a slide, but no manager is perfect. And if he was a bad manager, how come he won the Champions League?
But thats my opinion. Lots of people will differ, but I'm not going to say if they're wrong or right. It's just opinion