I got a full front and rear set for £35 from Katec/Keasea on ebay (they were EZO bearings) and they're all running smooth 6 months later, in fact I recently picked the seals off the bearinsg I could reach to check them and they were all uncontaminated proving the seals are working. Cheap bearings have poor tollerances but more importantly poor seals so get SKF, INA, FAG, EZO (from Katec)
This is how you change the front bearings, the rear are easy...
Pull the end cap adapters off which will leave the bearings and the inner half of the end caps still in place. Using a drift/screwdriver/socket extension etc and hammer knock out a bearing and end cap still in the hub by pushing the spacer between the bearings down to compress the o-ring so you can get the drift on the edge of the bearing then do the same for the other bearing. It's best to press the bearings in place but I used an old bearing on top of the new bearing and carefully knocked it in with a hammer, even if you do use a press make sure you push on the outer race of the bearing as applying too much force to the inner race will damage them. Make sure all the spacers and end caps go in with the o-rings seated correctly which can be a bit tricky if the o-rings have magically grown like mine had.