I've used some cheap ones off eBay. Same thing is common in the ones that I have tried.
Slight reduction in resolution aka things are slightly blurry. Also they have an odd feel. It's smooth but not as slick as a regular screen protector. Light output (brightness) is hardly changed if at all.
Personally I prefer regular screen protectors on my phone and non glare for my tablet. If there were non glare protectors that didn't impact feel or resolution I'd try them. However I suspect that these are tradeoffs for the antiglare properties.
Keep in mind I just buy cheap multi packs off eBay. I've only bought one brand-name screen protector and found it worse then the cheap ones!
Sent from my Nexus 7