Well, I originally made the mistake (or so I thought) of taking my two seven-year-olds to the cinema to see the new TNMT film, only to find that I quite enjoyed it (or did I just not hate it as much as I anticipated?!) I got hold of these three original films and have watched them with the kids a few times, and it has never seemed like purgatory. They are good fun, with several decidely funny one-liners thrown in in each film. The sets and costumes are obviuously very 90s, but if you go into these films with an open mind you should enjoy them.
The messages about 'family togetherness', 'respect for teacher', and 'good vs evil' are prevalent trhoughout, but in this day and age that isn't a bad thing.
I have to say that I was surprised by the quality of the turtle costumes and the actors/dancers/gymnasts inside them - give them a go...your tiddlers will love them! Oh, and the new TNMT does actually follow on from the third film of the series (if you check out the relics in Splinter's collection at the end you will see something from the third film therein!