It wasn't a mistake to move the TNG franchise to the big screen, as TNG was fizzling out and I think it's good they ended on a high note. The mistake was not in deciding to put the franchise on the big screen, but in how they did it.
Generations just wasn't well thought out and the plot really made no sense. A climax of three old guys wrestling on a mountain top just falls flat too. Not to mention Kirk died like that? I can't blame Shatner at all for resurrecting Kirk in his novels.
First Contact... well it was the high water mark. Still a lot of plot holes and inconsistencies, but this was the franchises best attempt at pulling off an action flick. Though turning Picard into an action hero just isn't convincing.
Insurrection... another plot that makes no sense at all and just has the feel of a TNG episode... and not a very good one. But hey the Riker/Troi fans finally get something after over ten years.
Nemesis... and the final nail in the coffin. Wonderful close up shots that just show us how old our heroes(and the franchise) have gotten. Everyone in this movie is just dumb... or the plot makes no sense, take your pick. The graphics and effects were great, but even the Enterprise and Scimitar exchanging fire for... 30 minutes... gets dull.