Criterion does a lot of good work, though they are not flawless. Admittedly, i have seen a lot more of their DVD's than i own, thanks to friends and rentals. They have made a lot of films availible here in America that otherwise are difficult as hell to find, such as Fassbinder's "Berlin Alexanderplatz". I try to buy their films whenever i can.
From my point of view the director whose work Criterion has probably best served is Ingmar Bergman. They have a great selection of well-produced DVD's of his films from all over his career, although there are some important ones that MGM has done pretty well with, too. I've been really pleased with all of these, but my favourite release that i've seen has been his so-called Faith Trilogy. These films looking stunning.
They have done a really nice job with some of the 1960's films of one of my fav. directors Jean-Luc Godard, apart from a couple of barebones releases like "Alphaville". But it is a puzzle to me why they have not done any of his amazing later films, many of which are hard to find here in America.
Back in the day, their laser disc editions of Orson Welles' "Othello" and "Magnificent Ambersons" (which have not been surpassed on DVD releases) were very prized and excellently produced packages, and are still collectors items. While it is unlikely that they will be able to release either of these films again, they did get to do 2 of the man's films, including "Mr. Arkadin". This film had an extremely strange cinematic release, being at one time availible apparently in around 10 different edits---and so for their DVD release Criterion was able to release one version of the film as it was originally released, one discovered some years later that was thought to be closer to the director's intent, and a newly reedited version based on information availble about what Welles had envisioned. While some consider this a somewhat minor film, the care that Criterion put into this release is wonderful, and these kinds of multi-version releases are some of the best work that Criterion has done, a la "Brazil" and "Fanny and Alexander".
It would be nice if they had the opportunity to work on more silent films. And certainly some fans of more obscure films have to be saying to themselves sometimes, "What? They're doing another fucking Godard !!?". I find the special features of their films less important than the beautiful quality of the releases---and yes, they can seem expensive sometimes. But as it is they have done a really nice job overall. Right now i really am wanting the Eric Rohmer 6 Moral Tales set.
BTW Luis Buñuel kicks ass.