I have an OU28T, which I've had for about 8 months, and I think it's a real bargain. Initially I was reluctant because of the low price, but there's a review here in the reviews section that tipped me toward buying it.
It seems decently built, and it does have a lifetime warranty, so if something major goes wrong, they say it's covered. I love the sound (8 strings don't necessarily fit every situation, but sometimes it really adds to the song. Intonation seems fine and the action is very playable, straight from the factory.
I couldn't put it down for the first few weeks, then we went on a trip and my son stayed at the house, and first thing when we got back he started raving about the 8-string, and how much fun he'd had with it. A good friend whose musical talent leaves me drooling came by, picked it up and sounded absolutely great (within a week he had ordered one of his own).
For the price, at least here in the US, it's hard to imagine a better deal. I can't comment on the Kala or the Lanikai, having never used their 8-strings, but I certainly wouldn't be afraid of either brand, I just don't know how well they stack up against the Oscar Schmidt.
I wouldn't want an 8-string as my only uke, but they sure are fun and add a pleasant and different sound for either picking or strumming, as you probably already know from your 12-string.