I'm using those dirty cheap screen protectors from eBay, usually US$2or $5 for a few, shipped from Hong Kong, on my 5800XM and N900.
I first tried the "invisible shield" on N900 to see if it's better than the cheap one on my 5800XM.
Big mistake! the "invisible shield" has orange peel look make the screen looks crap, also it has a little sticky feel when moving your finger. I just went back to the cheap, very smooth and good looking

Thank you all for your feedback