i had a review up here at one time but it's badly outdated.....
gamelauncher is basically for a nice, clean, simple fe....
Aos is for dos users that have an arcade monitor
emuloader is for people that don't care that their fe looks "windowsy" and wants to be able to configue game settings withint he fe. (and much much more in terms of configuration)
mamewah, and all of the gdi (read no hardware acceleration) fes are for setups running windows that want a lot of options, both graphically and configuration wise with moderate ease of setup.
DK, EmuTron, and 3DArcade are for the "big boys"
This is not to say they are better, but they will require a slightly beefier pc and are noticably more difficult to setup than the rest.
They are not for beginners (unless they are VERY dedicated), but the reward is a fe with far more options and prettier graphics. Also all 3 will take advantage of your computer's video hardware, which may or may not mean anything.

What it usually means is you need a decent video card to run them well.
EmuTron can do more than any other 2d fe graphically speaking. It uses a scripting language, so you can do a TON of stuff, but you have to figure out how to "code" your options for now. It's hard to setup but that's the price you pay for all of those options.
Dragon King Is mine (so I'm baised and you can ignore this section if you wish)
Right now it's in the very early stages, but I hope for it to become the 2d version of 3darcade. You will find the configuration files to be moderately hard at first, but as they are well commented it's easy to find stuff once you get used to the files you need to edit. It doesn't have as much "event driven" graphical freedom as emutron, but it's much easier to make skins for (at least currently, pacmanfan tells me he is working on some cool tools to help users out.) Dk supports better filtering options and multi-emulator support than emutron for now, but I dont' know how much longer I can make that claim at this rate.

3dArcade is in a class all by itself and is totally different than all the others.
3dArcade is also hard to setup, but your reward is the only 3d front end of the bunch. If you want flashy then you can't go wrong. It also has a 2d front end (what used to be called emulaxian) included so it is practical as well.
Hope that puts everything into perspective. Again these are just my opinions, but those who have used all of these fes would probably give you similar comments.