I ordered a pack of screen protectors for my 400D (from a german seller on eBay) and they do the job just fine.
Visibility is not affected, though I have one very small air bubble, and it wasn't too difficult to apply it (the seller provides precise instructions). At a couple of quid for a pack of 12 it's a no-brainer really.
I would imagine that some of these protectors are better quality than others... luckily I seem to have bought some good ones. They were also a perfect fit, so obviously made specifically for the job.
The ones I bought were
these.