Actually, I find the ads on the 360's UI surprisingly unobtrusive. They've done a pretty good job balancing this issue imo.
+1....I've actually clicked on a few ads (movie trailers) out of curiosity and purchased a few XBLA games that were on sale which I wouldn't have remembered/bothered to buy. I think it would be pretty cool if they had flash sale ads for MS points.
On a related note the Microsoft Store's pre-order deals are very nice...given that I would not have pre-ordered Halo 4 at full price without the $25 voucher. Most retail stores only gives you bonus content but MS gives you the content AND a $25 voucher. The store is in "My Pins" in my dashboard now. They have pre-order deals on non MS published games as well which I think is very generous of them.
With that said...when the hell is SmartGlass going to be released on iOS???