I bought a Lay Z Spa St Moritz in September. It's one of their larger models for 5-7 adults (allegedly). Ideal for 4 unless you're VERY close friends!
It was great, very hot, in fact we found 40c to be too hot. The air jets are nice but they are pretty noisy. I'd recommend boxing the pump if you can. We also bought the light, drinks holder (essential!) and the floor protector, plus all the chemicals.
The water requires a bit of monitoring to keep on top of chlorine/ pH etc, but it's easy enough with the starter pack of chemicals you can get.
The tub itself seems pretty robust, although long term it'd suffer from UV degradation I imagine. We had to repair a puncture in the lid (also inflatable).
As it happens we are selling ours now that we're moving house, although I think a mate is buying it from us. If he doesn't, then send me a PM of your location and I can send details.
The running costs are not cheap. Our electricity direct debit was increased by about £60pm. You want to keep it at around 25-30c so that you can warm it up quickly when yo want it. It seems to increase in temp by about 1c per hour, so a degree of planning ahead is advised unless you're happy to swallow bigger bills to keep it running all the time.
Overall I'd say they're a good entry in to the hot tub world, cheap upfront, not the cheapest to run but as lots of people aren't spending any money on committing any more, I can see why they're popular!