I had a 2002 280 pro, bought from new and kept it 18 months. I had no end of trouble between the 6 and 9 month period (partly due to a very wet trial part filling the airbox, then a faulty fan unit spinning at half speed), then nothing for 10 months, then gearbox trouble.
I loved to ride the bike, but I wouldn't buy another one until it actually seems like GasGas want to fix the obvious flaw. The airbox with a hole for the water to leak out isn't good enough for the UK weather as far as I'm concerned.
I considered myself a GasGas man, but bought a new Sherco 290 last week, so we'll see how we go. Performed well this weekend in pretty bad rain, and the airbox is bone dry.
These are the things that put me off the 2002 GasGas. Other people here will know if they're sorted on the 04's. I fancied a change:
Airbox design
Silencer design - rub through (now dented to stop rubbing)
Mid silencer - poor quality (now sorted)
Poor clutch - I had 3 in 18 months
Very expensive plastics (seat unit, rear mudguard and decals around