Being nineteen years old (too old to be a child, to young to be a parent), I might not be the best person to review this. But I was a huge fan of the books and television series when I was young (still am, I suppose), so I went to see this movie out of nostalgia.
Frankly, it's a bit of a mess. The story is that Sir Topham Hatt is away from the Island of Sodor on holiday and Mr Conductor (Alec Baldwin) is taking over for the time being. Meanwhile, an evil diesel (named Diesel 10- what happened to Diesel 9, I want to know) who, for some reason, has been built with a hydraulic grab, is planning to take over. Also there's a girl called Lily, her grandfather, Mr Conductor's cousin, a mysterious lost engine with a Magic Railroad, something called gold dust which has some kind of magical properties and is running out... and it goes on like that.
First point, the books upon which the original series was based were always grounded in reality (all right, a reality where trains can talk, but still). That is totally abandoned in this film- Sodor is no longer an island off the coast of England, but some sort of magical realm which can only be reached using a lost magic railroad or "gold dust" (which doesn't explain where Diesel 10 came from or, indeed, where Sir Topham Hatt went on holiday). Apart from the main characters, there are no humans at all on Sodor. The new characters are totally wrong for Thomas and the old ones have been rewritten and sugared to the point of nausea.
Other flaws? Well, if you live in Britain, you'll have no idea who half the characters are. Over here, we have no Mr Conductor, Stacey Jones, Billy Two Feathers or any of the other Shining Time characters. Also, as I said before, it's far too complicated. It's almost entirely padding, in fact. And there are too many plot holes, too many to list without ruining the film. And whereas some TV franchise movies rely too much on knowledge of the series upon which they were based, this movie keeps referring to events that haven't even happened (characters talk about Diesel 10 being "back"- when was he there before?).
The acting is pretty horrible. Peter Fonda makes the model engines look emotional; Mara Wilson is just confusing, looking twelve years old and acting like a four-year-old; Britt Allcroft's thankfully brief voice-over as the lost engine seems to be just an ego trip. The engines speak in a variety of awful pseudo-Brit accents, ranging from Percy's bad Cockney to Toby's bad Alec Guinness impression.
Strangest of all, given that he is nominally the star, Thomas seems to do nothing for the plot. He is basically an assistant who looks on while Peter Fonda, Mara Wilson, Alec Baldwin and Diesel 10 move the plot forward.
All in all, small children will enjoy it, but everyone else will be praying for the pain to end within half an hour. I must admit, though, I did like the bit when Diesel 10 was threatening to hurl Alec Baldwin off a viaduct. Summed up my feelings perfectly.