I don't think the series ever really jumped the shark - that would imply that the series became "silly" or had reached a point where suspension of disbelief was difficult. I do think the show reached a point - sometime early in season 6, IMO - where it did become more "safe" and predictable than it previously had been, but that's not quite the same thing.
I do think some of the characters jumped the shark, though: witness the Doctor's transformation from an intriguing and likeable curmudgeon into brain-dead comic relief, for one. Some characters that had rarely been well served really faded into the background: other than the last-minute romance with Seven, who can remember Chakotay doing anything interesting after One Small Step? I always think that was a case of the writers punishing Robert Beltran for daring to critique the way his character had been used.
People complain about Voyager being turned into the 7-Of-9 show, and they do have a point: but quite honestly, by the end of the series, she was, by far, the character with the most potential for interesting storytelling. No wonder the writers used her so often!
For all its shortcomings, I can't dislike the show, and always enjoy watching it. In fact, last month I started watching the show from the beginning for the first time since its original run. I'm up to season 3 now and am thoroughly enjoying it. The first season in particular was/is a lot better than I remembered it being.