There are 3 analogies for why I think X3 was a decent movie (and most of all didn't feel like last summer's Fantastic Four).
1) X3 is like seeing a reunion rock tour where a key member isn't there. The orginal members are solid but something crucial is missing. In this case Bryan Singer.
2) It's like when KISS released a disco album in the 70's. It was an alright disco album but if you were looking for KISS music, it wasn't there.
3) Ratner couldn't screw-up with Singer's franchise (cast, tone, art design, script outline) just as Bush couldn't lose in Afganistan with Clinton's army. The miltary exercise succeeded, but not as well as it would have under the previous leadership (Clinton would have gotten UBL). (And yes, as this analogy goes, if Ratner were to direct an X4-6 it would be like Iraq.)
As this post touches on some sensitive subjects, please forgive me if it offends any true beleivers or KISS fans.