Having not read the book, my general opinion of this film is certainly higher than many of the other reviews here. I personally found it was very well paced, had very nice and well choreographed action sequences, and had a decent (if simplistic) story. It's based on a book by the same author who wrote Jurassic Park, and in fact the composition of this film seemed to be very similar to that one (substitute dinosaurs for medieval knights, genetic manipulation with time travel, and there you go).
Unfortunately, just like Jurassic Park, the characters are fairly one-dimensional and the plot is very straightforward. One failing of this film that Jurassic Park didn't have is the science (or lack thereof). The word 'wormhole' is mentioned a couple of times, but that's about it. I can understand a director/producer not wanting to confuse the audience with unnecessary jargon, but in this case where the scientific theory underpins the whole focus of the plot (e.g. getting there, and getting back), more time should have been taken explaining the machine and how (and why) it does what it does.
The acting was fairly decent for the most part, with Connolly, O'Connor and Friel giving the best performances (as you might expect). Unfortunately, I thought the main lead (Walker) was less convincing, but this is probably more to do with the weak characterisation than the actor's talent. The 'bad guys' as a whole could have been played considerably more menacing; I think Hollywood has finally past the point where just having an English accent is enough for the audience to tell that a character is supposed to be evil!
One thing missing from the film is a sense of humour. Once the main characters have gone back in time, they spend pretty much all their time hiding or fighting - the odd humorous line or moment would certainly have helped raise the tone of the film, which is strangely rather dark. It's almost as if the film makers pitched the film at the wrong audience, this should clearly have been a family action/adventure film like Jurassic Park or Pirates of the Caribbean.
I actually bought this film because its music is composed by Brian Tyler, who wrote the music for the 'Children of Dune' mini series. Unfortunately, the score to this film isn't nearly as original or atmospheric as that, which was a bit disappointing.
The DVD itself is also rather disappointing. It has no special features at all (not even a trailer or cast bios) and doesn't even have subtitles.
This is a fast paced, medieval action film. If you liked Jurassic Park and like films with knights, catapults and plenty of sword fighting you'll probably like this film. If however you expect a sci-fi film or a tense thriller (which from the trailer, you would have assumed this film was) then you'll be disappointed.