All three movies
I have casually watched the series in the past. Recently, I casually watched a few reruns and started really getting into Futurama. I saw some of Bender's Big Score on TV and then watched the whole movie on DVD.

Of the three, I loved BBS the best. There were a lot of funny jokes and I love how they complexly interwove time travel into the movie. When you watch with the commentary on, you learn of all the thought that went behind this movie (e.g. numbers in the background, how they came up with equations, etc.) which they didn't do in the subsequent two movies. Couldn't watch the whole Hypnotoad episode. Zzzzzz. 5 stars.

The Beast of a Billion Backs was an interesting story but I personally didn't think it was that funny. 3 and half stars = 4 for the story, 3 for the (lack of) laughs.

Bender's Game was funnier than BBB but I'm not crazy about parodies because if you've missed the parodied movie.... In this case, it was the whole Lord of the Rings trilogy. 4 stars.

Still, I am great fan and will purchase the 4 season episodes.

Not Great, But Not Awful...
I'm a Futurama fan of a few years... watching the series got me more interested in learning about quirky quantum physics, and other oddities that made the show a gem in the world of animation.
Loved Bender's Big Score... it hearkened back in a small way to season 4, with the exploration of the Fry/Leela relationship. And I like a little drama combined with satire and comedy (and insane characters). Adds depth to the characters, and the show, in my opinion.
That said, I didn't like BWAMB nearly as much as BBBS. It destroyed all the emotional buildup of the end of the original series and the first movie. That threw me off my game in a big way. Also, I would have liked more of the witty zingers and quotables I grew accustomed to with the show.
And I know a lot of people complain about the giant tentacled Yivo. If any of you ever watched a few genuine 1950's B movies, you'd understand why Yivo is there. The DVD cover also makes more sense when you connect BWAMB to movies like The Crawling Eye, and other wacked out sci fi movies of the late 50's. I do admire the Futurama crew for what they attempted here, in lampooning the B movie genre. Sadly, it doesn't measure up to their standards established by the show. Nor does it go far enough in its satire of b movies.
I hope Bender's Game will be an improvement. I REALLY hope Into The Wild Green Yonder surpasses even that. I've grown rather attached to all the characters, and hope Fry and Leela end up together, and end the movie series (and possibly the entire series) on a happy note. Is real closure too much to ask for?

ambivalent
nothing smart or funny about this one. very disappointing for a futurama fan. i'll forget this one exists..

Futurama: The Beast I Should Not Have Purchased
Futurama: The Beast with a Billion Backs A disappointing DVD. Not up to the standard set by Bender's Big Score or the TV series. A few good lines, but weak & sappy plot. Bought for my daughter--two weeks later it's on TV. Even the October stock market would have been a better place to put the money I spent on this DVD.

Eh, It was ok
Have you ever watched a movie and fallen asleep during it and not cared...Or after a movie that you just watched you feel eh, ok, but nothing really sticks with you. After Bender's Big Score I was hoping to be awed with a tight story, Bender's usual hijinks, Fry's longing for Leela, and Zoidburgs classic quotables...Instead, I feel cheated, it seemed like whoever wrote this movie had never seen a Futurama episode before, never mind even written one. I really am hoping that the next movie is better, because I really want Futurama to go out with a bang.