I bought one a few years ago, shot real close to 500 fps. Took it apart to clean, smooth and lube it and the spring had quite a bit of preload.
Got another one a while ago on a rainy day just for something new to tinker with. It shot around 250fps, could watch the pellet fly. Took it apart for a look see, figured it was a bad seal. The spring was way shorter, with no preload. Very unhappy with it so I returned it the next day and got a Beeman P17 instead as I knew it was a decent gun.
Best value these days for cheap guns would be the P17 for a pistol and the Ruger Airhawk (if you find it for around $100, even at $150 it's a good gun) the Airhawk is a chinese clone of the Diana 34, a very nice clone!