"1 is it a good player most ... turn out some rubbish?"
-> I think since they've been hit by Apple and other MP3 players, they've refined it, and made it better. HD3 is a good device, I would say.

"2 if i download music from the net will i be able to put it onto it?"
-> As you suspected, there's format differences. When you download from Apple iTunes, they will only play with iPods, and if you download from Connect, they will only play on Sony device.
But there's a work around to this. Most online services allow CD burning, so what I do is for backup purposes, I always burn them to CDs. Once on the CD, you can always re-rip into whatever format you want, like MP3.
Then you can play them on whatever player you want.

Frankly, I'm a HD3 user, and I love it.
Sound quality of ATRAC3plus (64kbps) is great. When you get to know it. And the battery lasts forever on this guy.

Hope you love your device, whatever choice you make.
Cheers,