Not 336x280 but I do use 728x90 leaderboard and they suck. Probably because I stick them into the space where everyone expects to have a banner ads and ad blindness kills it.
I think 336x280 works for realcartips because he uses very large font for the rest of the content. It kills two birds with one stone, so to speak: the page appears to have more content than it actually does and the huge ads no longer appear huge. Some (most?) of his money pages are extremely thin and even actually scraped (not to mention old - there's nothing I see that's younger than 2011) so I can see where he might want to make them look bigger with large font.
I also think large font makes his pages more mobile phone friendly - don't know if it plays a role or not but 336x280 ads are still useable as mobile ads due to large font sizes and all the rest of the blocks (I haven't tried the largest yet) are simply useless for mobile traffic, at least in text format.