Finally took the plunge on one of these, had to order a US 4.5 3DS XL which are easy to find in China, and the seller also had stock of gateways too.
Initially I was apprehensive as it sounds a bit half cooked and its very easy to accidentally update your console and therefore block the use of the GW 3DS.
Anyway, having the device in hand and being able to see how shit works, it really isn't so daunting.
Plays almost all current 3DS games (Tomodachi Life beaing hte only notable one)
Free games bitch!
Plays foreign games.
Plays eshop games (same as you would a rom)
Homebrew (although little exists to date)
Really not that expensive when put into perspective.
Reliant on the 3DS having an ancient firmware
Risk of updating bones you royally.
Nintendo may find a way round it, and bone you royally.
Saving your game requires certain steps to ensure it works.
I'm not going to go into great detail as there is plenty of sites that reviewed it months ago, but the operation once you get your head round it is simple.
Basically, you run the blue DS card and you don't need that card again until you either want to update the gateway, or you have played an old DS game as DS mode resets it.
From then on, you need to tap settings and go to open your DS profile, which activates the hack, the 3DS boots into haxx0r mode and then will boot games off the gateway 3ds card. This setting > profile needs to be done each time you power up the console. it often reboots back into the original mode without you noticing, usually if you are poking around in the 3DS settings.
You have an emunand mode, which I haven't fully explored but basically you can copy your current NAND (something you should do anyway and back up to your PC) and when you boot into haxx0r mode you will be in the emulated nand running from your SD card. Now you can update this to the latest version of the 3DS menu system and then play games which need a higher firmware than the 4.5 on your 3DS.
Personally I haven't done this as Zelda (and every other game i have tried) is working without doing it, and there's the risk of fucking it up and accidentally updating your system NAND to the latest one, which I know is slim but at the mo I have no reason to do it.
Backing up your NAND means that should you make the mistake of updating, you can do some relatively simple soldering and reflash back to 4.5, using your PC so thats a nice safety net, but I believe this only applies to the 3DS XL.
This is by no means a tutorial, so there are other set up steps but nothing out of the ability of the average DFer.
There are a number of other cards out on the market now, both are cheaper and both are clones of the gateway, I chose to spend the extra and support the devs. Since Gateway released their V2 'Omega' firmware it seems they have stopped their software from being copied for the clone cards and as this is the case, the competition is lagging behind significantly since Omega bough homebrew, eshop and a ton of game compatibility.