I didn't take to Spy kids 3D as I have a lazy eye and so the 3D effects were literally a pain - I liked the film otherwise though. Fortunately the 3D effects in Shark boy and Lava Girl seemed far less annoying at the cinema and the fast moving film was great fun for my son (9) and I - mum and daughter didn't want to see it - they Nanny McFee'd instead. The film is essentially one long fantasy day dream sequence that merges with reality (and as its complete fiction anyway it hardly matters whether the action is 'real' or imagined). Although having a very similar feel, the storyline in this film is probably a bit stronger and more original than Spy Kids 3D - which was rather like TRON and Scooby Doo & the cyber chase (no bad thing actually). Shark boy's not unlike the 60's 'The Phantom Tollbooth' in a way or a 1980's Dr Who serial, but the special effects are naturally first rate.
I must say that, although always visually startling and stylish, it does get a bit repetitive towards the end for adults, but my son really loves it and has happily re-watched the film a few times already (and that is rare with most of his DVDs). I suppose the plot is a bit childish (hardly surprising being written by one for others), but the early part of the film is really good, particularly the first few minutes recounting the genesis of sharkboy. Besides many movies these days have a "beat up the little vampires first and take out the big bad super vampire at the end' plot, so this is a tiny bit different - although it's still the cosmic battle between good and evil. I'm not going to give away any of the plot though.
Despite the 3D glasses, overall I found the film very entertaining. Fortunately this DVD has both the 3D cinema version and the 2D normal version. In fact my son also prefers watching the 2D version on TV and he rarely goes for the 3D version unless it's with his mates and popcorn. You get a his (shark boy) and hers (lava girl) set of green/red 3D glasses included. So if you have a preteen boy about the place who missed this at the cinema, get this for him now - he'll thank you for it. So childish, but highly recommended.