I don't have one, but about a month ago I seriously auditioned a K28e during a quiet weekday in my local store. IMO the extra bass from the large GO body partially offsets the bright koa tone, which tends to be a bit light on the low end - at first. (Koa tone is known for opening up in the lows as the guitar ages and gets played in). I came home and immediately played my GS-K, and liked it better. The 2006 GS-K *is* the best guitar that I own so the bar is set pretty high. GS-K is now eight years old, but has a spruce top instead of the much prettier koa top and fancier fretboard inlay scheme. I also remember playing a 618FE as soon as they came out (my first exposure to the GO body) and was a bit underwhelmed. Absolutely nothing wrong with it, but the tone and balance was not quite to my taste.
Maple tone is overall fairly close to koa tone, so if you really like the 618, the K28 will likely be a winner for you, and much more spectacular looking. I also own the 2007 424-LTD (all koa) and like it a whole bunch. It surprises me pleasantly every time it comes out of the case, both for tone and visual beauty. I really should play it a lot more than I do. Never got to play the 2011 LTD's, so no opinion there.
With any koa guitar it is vital to play before you buy, as there is quite a bit of variation in tone. As a broad generalization, my experience is that the more highly figured the koa top, the more likely it is to be tonally anemic or even a dud. Less variation for koa body / spruce top. if you cannot play it in advance, at least make sure there is a good return policy. At the price point of a K28, it would be painful to be stuck with a guitar that expensive if it does not truly speak to you.
