I used the Fellowes WriteRights when I had a Palm. Go ahead and use them. They won't harm your screen, but they are poor quality. In a few days you will notice some worn out, scratched areas, but your screen will not be affected. If you don't want to spend more money, the Belkins are priced about the same and are a little better.
It's easy to recommend the premium screen protectors (Boxwave, Writeshield, Brando, etc.). Honestly, even though I use the expensive ones, I don't think I can justify their cost. You will spend less money if you use the cheap ones. Premium screen protectors are more of a luxury than an economical way to protect your screen.
I'm using the Fellows right now, with the same complaints as above posts. Too thick, scratch easily, and not the clearest out there, BUT hard to beat the price, and is definately better than nothing. Use them until you can get something better, save the left over ones to cover you while waiting for the next order of your preferred brand to show up. Have seen some torn up PDA screens that had no protection. Spent too much on this toy to not protect it!!
Im still using my box of 12 from 4 yrs ago. I hate them they are soft and mushy but they do the job of protecting the screen I havent had any scratch though. I am trying to decide which ones to go with and as soon as I do theyre back in the drawer for the next better'n'nuttin occasion.
If you already bought them, then go ahead and use them. It shouldn't harm your screen in anyway. When you are done with them, consider some of the brands mentioned here. Also check out the accessory forum. Plenty of info about screen protectors. Screen protect should not only protect the screen surface but also give you a decent writing surface. I used Fellow's screen protect way back in my Visor day. Well, let's just say one needs to start with a reference point :D.
I use Pocket PC Techs' WriteShield C2. If it lasts as long as the previous WriteShield I had, I don't expect to replace it for another 12 months, minimum.