The Kreg does the job properly: You have to allow for different thicknesses of stock (meaning the holes enter the work at different distances from the eventual mating surface). To do this the jig needs to be height-adjustable, with respect to the clamp. And it needs to clear chips easily (to stop the drill overheating).
Kreg or UJK. Both are well thought out. My
Kreg kit came with a good instructional DVD, and I'm extremely pleased with it. It's quite a lot more than you want to spend, I appreciate, but I wouldn't swap it, and I'm glad I stumped-up. You can also use it just clamped-on (without the clamp part of the jig), but that does reduce the accuracy a bit.
When you assemble stuff, the joint does need to be held rigid in the right position, otherwise the angled screw can pull it out of alignment pretty easily. I'm sure you don't _need_ the special clamps (I don't have any), but you do need some clamping arrangements of some sort. I've learned this the hard way!
E.
PS: The Kreg one has a good storage box too - good as there are small parts to look after. Dunno about the others though.
PPS: Whatever screws you use need to have a wide flat head on the underneath (screwthread) side, so that they pull on the biggest area of wood possible - countersunk screws will probably split the wood or pull through. Robertson screws (square-headed drive) are the nicest to use, by far, with Torx a close second. Most other types will cause frustration when assembling and trying to get a screw into an awkward slot.
