He's not a world class manager, but there are very few of them. In my opinion he's a fairly good manager who could get a team like Chelsea to finish in the top three on a regular basis and also give them a reasonable chance of making the CL semi finals or quarter finals. There are a number of other managers who could probably do this (maybe Mancini, maybe Cappello) as this is the minimum you would expect of a squad like Chelsea. With a better manager they would probably have a better chance of winning big trophies, but there are very few stand out candidates for them, maybe Mourinho, maybe Hiddink. Even then, there are problems with the set up at Chelsea (with people like Gourlay and Buck insisting on playing players like Torres and adhearing to a nonsensical idea that players over 30 (Lampard, Drogba etc) should leave) and no other top 3/4 team in England has to deal with these sort of issues. Its worth noting that other reasonably high profile managers have struggled at Chelsea (Villas Boas and Scolari had some very poor results there too). You go on and on about Benitez not being a top class manager, but you always aviod suggesting a credible alternative to him. For whatever reasons they got rid of Di Matteo a few months ago and at the time there were not that many alternative candidates with Benitez's experience.
You say that his CL win was lucky, but you could put that charge at a lot of teams who narrowly won the CL / European Cup. Were Milan lucky in 2003? Were Man Utd lucky in 1999 and 2008? What about Barca against Chelsea in 2009 (semi final)? Or Chelsea last year? In the end any team that gets to final almost always has to be several great teams in the process - so I think it've very disingenuous to write off all Benitez's European achievments (getting to several finals and winning some of them with both Valencia and Liverpool) as simply "luck" of a one off game in a final. In 2005 his Liverpool team played Juventus, Chelsea and Milan and saw off all of those teams all were faily high profile teams - the year before Chelsea had been in the semis and two years earlier Milan and Juve were the finalists. In 2007 they got to the final again and beat Barca and Chelsea (again) as well as a fairly good PSV team (who had knocked out Arsenal and two years earlier had only gone out on away goals to Milan in the semi final. I don't think you can have a situation where beating all these teams is just down to "luck".