It's not the popular opinion (see the unpopular opinions thread!) but I think Missing Ace is pretty good. I prefer it to any 2000~2008 Rider movie except Hibiki's and Saraba Den-O.
I think you sort of have to view it in the context of what it is: a sequel to the end of the series long before they really had the end of the series planned out. It's more in-line with how the series played out early on I think, in regards to how the Kenzaki/Hajime thing works (Up to a certain point, even when they were kinda buddy-buddy, Kenzaki still would have sealed Hajime if he absolutely had no other alternative. By the end of the TV series, that becomes almost unthinkable. The movie basically works on the premise that during the final battle, Blade did seal Joker- and to its credit, it doesn't treat that like it's a great thing.)
There are some things about it I prefer to the TV ending, and there are some things the TV ending handled better. Having more time is the big one- Missing Ace feels pretty rushed. There are some awesome ideas that just fly on by, and I defy anyone to explain what the hell's up with Tachibana in most of the movie (although if you look at it as him working an angle to try and draw the villain out, it makes a lot of sense, right down to his plan getting screwed up because Tachibana is a lovable loser.) There's a lot about it that appeals to me though, particularly how it gets into the idea of life-after-series, which TV Rider rarely does. The classic shows kinda did it with their own Tachibana and some of the Riders, but that's about it. Even in the sequels of Den-O and Double, it's more like "more adventures!" rather than any kind of serious examination of how fictional characters are defined by their series and without that series, they just sort of drift and pine for the good 'ol days. Missing Ace sorta goes there. I've probably gleaned a lot from very little, but I really like what it has to say.
It's definitely an acquired taste though. Most people don't like it, so don't go in expecting to. But I think it's got some seriously good stuff going on in it.