I'll let you know...
...I just bought a grey Oreck XL on eBay, primarily to try one out, since I was also curious about them, then to sell it on. Hasn't arrived yet, but I'll report back when it does.
As I understand it, they have good, higher-than-normal-speed brush rolls, which are great with surface litter. However, with tiny fans, the suction's not brilliant, and being so light, they're hopeless at deep cleaning. I've read reports of them getting through belts very quickly, too.
I think they're hideously ugly - just my opinion, though. Very functional, no concession to aesthetic appeal - to keep the weight as low as possible.
They're popular with the elderly, since they're light, and make the carpet look good. You always get ads for them in Saga Magazine! I guess their traditional, bag-on-a-stick look is less threatening to older people than something like a Dyson!
Haven't seen anything which justifies a £400 price tag, though. Hugely overpriced - again, in my opinion.
Here's an interesting video I found on YouTube, comparing the Dyson DC07 and the Oreck XL7: