I've always hated keyboard covers with a passion until I bought a used MBA that came with a brand new Moshi cover. I gave it a chance and eventually I got used to it. The way I type tends to wear out keyboards so it prevents that too. The specific type of material the Moshi is made out of is a night and day difference to the crappy silicone ones on ebay. It has a non-stick texture that ensures your fingers dont stick to it. I feel like typing is "weird" now on my laptop if I don't have it on. Typing is also just as fast with it on or off.
I did a test yesterday and it makes no difference on the temp of the laptop. I had my MBA in clamshell mode driving 2 thunderbolt displays and iStat monitoring it. There is basically no difference between having the lid closed or opened or having a keyboard cover on or not.
The cover does affect the speakers a little. It seems like high pitched sounds are blocked but the actual volume is not affected in any measurable form.
I say give it a try and just return it if you don't like it. It really does take some getting used to but might be worth it if you can overcome the different feel.
Thanks for the review. Do you close the MacBook lid with the cover on?