WARNING: As a huge 3D James Cameron informed us prior to the clips, the 15 minutes being presented today are taken from the first half of the movie and shouldn't spoil it. If you're less sure, better hit the back button.

Having just emerged from the IMAX theatre on London's South Bank, from one of many 'Avatar day' showings of 15 minutes from the new James Cameron movie, I still don't really know what to think.

My initial impression of what I have seen is that Cameron is easing his fans into his 'fantasy' era by throwing a bit of Aliens/Terminator technology in to top and tail a type of movie that fans weren't necessarily expecting.

Listen up, maggots...
The first scene will be familiar to anyone who loves Full Metal Jacket or Aliens, as drill sergeant-type Col. Quaritch (played by genre favourite Stephen Lang) informs the new arrivals on Pandora that they can basically expect to be killed the second they step out of the military enclosure.

Sam Worthington trundles into the briefing in his wheelchair as Quaritch ramps up the warnings:

"I will try to keep you alive - but I will fail. For most of you."

I've always been a fan of Stephen Lang, a most distinctive and familiar face in genre movies and TV, and he can commendably add himself to the roll-call of cinematic military hard-nuts, on the strength of what I've seen.

Weaver and Worthington
Next we see Worthington in Dr. Sigourney Weaver's lab, annoyed that she is trying to help him get into the huge coffin-like structure which will enable him to have an out-of-body experience in the huge, pan-like blue avatar (or host-body) that will finally let him walk again. Worthington and Weaver spar a bit before she finally closes the lid on the loud-mouthed recruit and sends him into some impressive wall-mounted technology clearly based on a CAT scanner. She also announces that she's going in herself.

Wakey wakey
Next comes the scene that made the biggest impression on me, even though it's the point at which we are being weaned off the wonderful hi-tech glory of former Cameron and into the world of the elves and the faeries: Worthington wakes up on the slab as a nine-foot blue avatar and against all medical advice, tries out his legs immediately. He's not even aware that he now has a tail to consider when making sudden turns, and consequently causes absolute havoc. His fellow avatar-inductee tries to calm him down, but Worthington won't be tranquilised or stopped from trying his new mobility on for size, and runs out of the medical complex, a huge but nimble kid determined to try out his new toy.

The CGI in this scene is probably about as good as the current technology can permit, particularly given that the VFX team is tasked with creating such a bizarre and unworldly creature. It really is impressive to see blue-Worthington get up and cause chaos. The complete scene gives a far greater impression of realism than the shorter excerpt seen in yesterday's trailer. The roving camera also adds to the sense of realism. This part of the preview was first-rate.

Insane bravado in bright colours
Before long we're with blue-Worthington in the jungle of Pandora, and the eye is verily assaulted by an explosion of colours; it's like an explosion in a paint factory. It's as if someone created the Pandora colour-palette from the shelves of Toys-R-Us.

Blue-Worthington and Blue-Weaver are facing up to a formidable jungle animal, a Rhinocerous-style beast with literally about ten feet of bone protecting its internal organs. Weaver advises Worthington that shooting it will only make it angry (does Mel Brooks get a royalty for that Blazing Saddles line?). The creature rushes Worthington, who does just about the most insane thing ever depicted in a sci-fi movie, and faces it off. Presumably our Sam got institutionalised as a cripple and is determined to return to that status pronto.

Well, the creature improbably backs off, but not, as it turns out, because Worthington is great, but instead because something even nastier is sneaking up behind our hero (and this creature is really too hard to describe, I'm afraid).

Lovers' tiff
Next there's a big verbal sparring match on the cards, as alien warrior Neytiri (Zoe Saldana) saves Worthington's arse from more forest nasties with some impressive spear-work, and then berates him for having lit a fire and attracted a whole bunch of predators towards him, obliging her to kill the forest creatures that she respects.

Worthington points out that she could just have let them kill him, and here, I'm afraid, is where Cameron's inability to handle romantic dialogue - a curse that has plagued the director through practically every movie he has ever made - returns with full force, and in 3D. In movies, as in love, it's all about timing - people don't just blurt out this stuff. Movie scripts are all about reductionism, and handling a love story within those constraints is a very special talent that Neil Simon shares with Woody Allen, and James Cameron fails to share with George Lucas.

Also, at this point, I'm beginning to wonder what the hell kind of a sci-fi movie this is anyway. Bright blue people, loincloths, brave deeds, campfires...pretty soon it's looking like Hiawatha In Jurassic Park.

Harryhausen territory
In the next scene, all is clear. This is not a science-fiction movie. It has more creative DNA in common with Edgar Rice Burroughs than H. G. Wells. Worthington and Saldana approach the mountain-top nest of a whole bunch of very fierce and faintly pterodactylic creatures (but far more powerful), in what seems to be some kind of rite of initiation for Worthington into Saldana's tribe, who are with them.

Before you know it our Sam has whipped out his own brand of utterly insane bravado and is wrestling one of the beasties, taming it, and being taken for a mad ride down the ravine and dashed against walls in a comedy (and very 3D) manner.

A ray of hope
As if to remind hard sci-fi fans not to entirely lose hope, the fifteen minutes of footage closes with various fragmentary scenes, most of which we have already seen in yesterday's trailer, depicting some huge battle at the end, which continues to look to me like out-takes from Attack Of The Clones, with some Matrix-style exo-skeletons thrown in.

I do note that one of the ships in this sequence, though glimpsed only briefly, is practically a recreation of the dropship in Aliens, and that was a welcome enough sight. But it can't muffle my disapointment...

3D
The IMAX 3D gave me a headache almost instantly. I'm a huge fan of IMAX 3D movies, and have never experienced that level of almost instantaneous discomfort through the undersea and space movies that they show at the BFI.

The 3D effect itself is, to be honest, inconsequential anyway. Yes, it works fine, but it still feels as if one is looking through a set of planes-of-action rather than a continuous recession into the distance. Cameron seems to use jungle foliage judiciously to remind us that the effect is there at all.

The 3D is fine. It's nice.

It's a gimmick.

It won't save Avatar, if it should turn out to need saving - a matter on which I will continue to reserve judgement.

• James Cameron's Avatar: the trailer is here
• The Avatar Trailer: Oh dear