It probably has more to do with personal preference than anything else.
Some people like Screen protectors that fit snug but have nothing adhesive or tacky that make them stick to the screen. Some people report that protectors such as those put out by Fellowes or WriteRights (Not to be confused with WriteSHields which everyone says are great) feel mushy and have too much give when writing on the screens with the Stylus, especially when using Transcriber or Block recognizer.
The complaints about the Dell Screen protectors that I have heard say that the tiny bumps on the undersides of the protectors leave small indentations on the screens during heavy usage. (I am not confirming or denying this claim as I have not used them, I am merely reporting back the comments I have read from other users who have used the Dell's).
And then there are very simple complaints such as length of time before the protectors themselves getting scratched or indentations made and need to be replaced, if the protectors have a glare to them (of course you would hope that the protectors would reduce, not increase glare), do they attract dust particles and does dust and dirt get trapped between the screen and the protector itself, are the protectors clear or do they give the screen a cloudy or foggy look.
All of these factors, complaints, pros, cons come into play when evaluating Screen protectors. And from pretty extensive research I have found that the ones most recommended are the WriteShields from Pocket PC Techs. Now keep in mind I have yet to receive mine as I just ordered them today but I will tell you that they are pricey (29 dollars for one pack which contains 3 protectors) but on the flip side most users report they need to change them every few months at the earliest. But its up to each person to make the decision about the ones they would like to use, that’s why these forums are so great - to get first hand experience from people. Not the company line.
link to Writeshields
http://www.pocketpctechs.com/ws.asp