I have 4 packs of GP4300 matched by Demon (UK matcher). These cells cost less than half the price of other packs available at the time I got them.
One pack was supplied with a 'bad' cell, but this was replaced FOC by Demon.
I have now been using these cells for about 6 months and they are going from strength to strength, really punchy and no run time issues at all.
I'm charging at 5.5amps (Orion Advantage,3mv per cell, linear charge, capacity set to 4500mah), once I have run the cells I just chuck them back in the box with whatever charge is left in them until the day before I next race. I them discharge at 10amps down to 6v and leave them till raceday. On raceday I put them on a Noval Smart Tray to bring them down to 0.9v per cell and then put them straight onto charge. I found this regime to really work well.
However I could be unique as it seems most matchers have now dropped GP4300 in favour if IB4200 - Matched GP4300 seem difficult to obtain now.