Considering that similar bikes are being sold in shops for nearly 700 quid I took a risk on this one from Bicycles4u and so far I'm more than happy. I considered a slightly bigger version from Bicycles4u that comes with suspension, but decided that cheap suspension probably isn't worth the extra money. I'm using this to commute to work, not to go tearing about on mud tracks.
It weighs about the same as the £700 bikes I saw i.e. while not weighing a ton you wouldn't want to lug it up ten flights of stairs, but if you really want a lightweight folding mountain bike then you'd need to spend WELL over a £1000 anyway. The pedals fold and the handlebars come out of the stem if you're absolutely desperate to fold the bike as flat as possible. Naturally on a bike this cheap the saddle seems designed to cut you in half, so expect to replace that. The frame, gears and brakes are fine. Like many mountain style bikes it comes without mudguards, but I found good ones for a tenner so that's no problem.
The instructions warn that the first few times you ride the bike expect to find bits on the bike that need tightening up and they ain't kidding, so make sure you take a set of alan keys with you! This is only a minor niggle as it doesn't take long to tighten things, and since then nothing has come loose.
So is it worth buying? On price alone definitely. It's in competition with similar bikes retailing in shops for £600-£700 that weigh pretty much the same and appear to offer no significant advantages (certainly not an extra £500 worth!). It's also in competition with Argos, who sell cheap folding mountain bikes for between £100-£200. But judging by the complaints on Argos's website those bikes appear to be very shoddily built, with brake mechanisms fitted backwards that have to be totally dismantled by the customer and fitted back on properly. All things considered I'd definitely recommend this bike over those.