The best word to describe this I think is fun. It starts off a bit dodgy and slow, but then as the series goes on, the storylines become more involving, and it's very good entertainment. On a scale of sci-fi goodness, I'd say it was at about the same level as 'Sanctuary' and 'The Dresden Files' - maybe a bit better. Towards the end you may realise you really care about the characters, even Artie the gruff Warehouse boss (Saul Rubinek), who I really wasn't keen on at the start, but I think grows on you.
It's also a really great idea for a show - a massive warehouse full of magical objects, or 'artifacts'. It will be hard for them to run out of ideas(yay, more seasons!), when there are millions of objects, and each one can be totally different. The episode I just watched included humourous but menacing dodgeballs that bounce up and down by themselves, and multiply on contact, or just downright evil historical artefacts, such as Edgar Allen Poe's notebook and feather quill, that inspires sociopathic tendencies and can make whatever it writes a reality.
The technology is the show is also a bit different, having evolved in the world of the centuries old Warehouse. For instance the 'Farnsworth', which looks like a rusty shaving mirror from the 60s, but is actually portable 2-way visual commmunication. Also pretty cool is the 'Tesla' - he worked for the Warehouse, obviously - which is basically a comical looking ray-gun, that fires bolts of electricity to stun the bad guys. Totally steampunky, and better than guns by a million. The visuals are normally great too, especially the sweeping shots of the gloomy interior of the Warehouse.
Warehous 13 also has more of a quirky sense sense of humour than your average sci-fi show (except Firefly of course), Eddie McClintock (playing Agent Pete) being particularly good at delivering the odd Whedonesque-ish line. However the wry, slightly teenagey humour does have a flipside, as it means that neither leads are particulary convincing as experienced FBI Agents, and because of the light tone of the show, the dangerous situations never really feel that dangerous. X Files it aint. Personally, I think it would be a little more believable if they toned the humour down a little bit, especially Agent Pete, and quirky 19 year old Warehouse wannabe Claire, who only speaks in ironic self-aware geek speak. It works really well though when theres a really serious scene, and then it's undercut by humour. In the same episode as the dodgeballs (I have a very short memory), Pete and Mika (his female FBI partner) have a long, serious conversation, which ends with Agent Pete saying dead-pan 'Also there's another problem. I think I'm hot for your mom.'
Also I think the show missed out on some opportunities. The agents got used to the physics-defying weirdness of Warehouse 13 too quickly, and also their relationship seemed to go from mutual annoyance in the pilot, to affectionate sibling rivalry in episode 2 or 3. They could've taken the whole season to get used to the supernatural goings on, and also to develop their relationship. There's zero sexual tension between them by the way, although that is actually quite refreshing as they still have good chemistry as close colleagues, who love each other in a friendly kind of way. From watching the special features, it does seem as if the actors and crew are all nice people who genuinely like each other, which definately comes across in the atmosphere of the show.
So basically, don't expect *too* much, but enjoy 12 episodes or so of humourous and intriguing sci-fi nonsense. If you're already a fan of light-hearted sci-fi, prepare to be addicted!