This device made the GBA worthwhile to me.  When it was announced I wanted it so bad I modded my Cube so I could import the Japanese one months before the American one came out.  Later I bought an American disc and, presto, my Japanese GB Player turned into an American one.

I don't hold still while playing games.  This is a big problem with handhelds as the controller and screen are one in the same so the screen bounces with the controller.  Another problem is the ergonomics of a handheld.  With a typical videogame you look forward at the screen while the controller is around waist level.  With a handheld you either have to hold your hands up in front of your face (which no one does) or look down, which fucks up my neck.  I get why one would deal with this while on the go since you have no choice but it's incredibly shitty for playing at home.  The GB Player let me play these great GBA games the way I wanted to, especially since I have no need for a handheld and only buy them to have access to the exclusive games available for it.  And it works with ALL Game Boy games as well so you can get GBC games on it, too!  My only beef is that it doesn't use the SGB palettes for the games that support it but that's okay because I have a Super Game Boy anyway.

My DS gathers dust.  I never play it because I hate using handheld systems (and I hate the touchscreen; the DS was a big waste of money for me unfortunately).  If the Wii U had a 3DS player (no 3D obviously but who cares?) that also played DS games I would be set!

Surprisingly this is somewhat of a controversial accessory as the "that goes against the point of handheld games" argument comes up a lot, which is strange since there is no reason one can't play them exclusively on a handheld if they want to.  What's wrong with the option?  And when you look at games like Minish Cap and Metroid: Zero Mission there is NOTHING that sets them apart as "handheld" games aside from being 2D.  Boktai?  Sure.  You couldn't really do that on a console but most handheld games have nothing that truly seperates them from a console game.  With the Wii U we'll see that even more as the unique features of the DS are put in a console environment.

I remember my friend and I connected two Cube's/GBA Players together and played Mario Kart Multiplayer with two TV's! That was pretty fun.

I'm pretty sure UncleBob has played either Four Swords Adventures or Crystal Chronicles (or both) with five GameCubes, four with Game Boy Players, and five TVs. I love UncleBob, because he makes me feel like I've made sane decisions about how much stuff I've bought from Nintendo.

Advance Wars has a single system multiplayer that just has the players taking turns and passing the GBA between them.  With the GB Player all of the Gamecube controller slots act as player one.  So my brother and I would load up Advance Wars on the GB Player and play with two controllers and just assume control when it was our turn.  It felt virtually no different than playing a console game... except with the ability to fuck with your opponent's controls which happened at least once. ;)

I never bought a GBA so this was a cool accessory to have. I remember buying Advance Wars and getting to the last level (before the cartridge disappeared), buying A Link to the Past (which went MIA for years until a month ago when I was cleaning my room), found a copy of Iridon 3D on a camping trip (and then lost it), and bought a used copy of Metroid Fusion (that someone already beat). I remember being sick one time and my mother was bringing me some orange juice that spilled close to my GCN and Fusion was stuck in the GB Player for years. Unfortunately I don't know where my start up disc is so I'll have to buy another one if I ever want to use it again.

I've done a few fun things with my Game Boy Players.

Final Fantasy: Crystal Chronicles - Though I've never played 4-players, I have hooked it up (5 GCNs, 4 Game Boy Players, 5 TVs/Monitors).

Four Swords Adventures - I have had four players here - 5 GCNs, 4 GBPs and 5 TVs/Monitors.

Four Swords + - Same thing, just in the Tetra's Trackers mode.

Wind Waker - Second GCN/GBP as the Tingle Tuner.... for no reason except I can. :D

Pac-Man Vs. 2 GameCubes and 1 Game Boy Player.  This one doesn't actually work as well - I always wanted to get one of those lazy-susans and sit the Pac-Man TV on it so it could spin around and face only one player.

Original GB Tetris - Did two fun things with this - Obviously, two GameCubes and two Game Boy Players.  Nothing special.  More fun was one GameCube, 1 Game Boy Player, 1 SNES and one Super Game Boy 2. :D

Four Swords - Only played 2-players more than once.  Played 4 players one time.

Mario Kart: Super Circuit - Same as above.

Super Mario Advance - Mario Bros. Classic mode - same as above.

Animal Crossing: You know that cool mode where you can download the NES titles to your GBA?  Works on the GBA player too, if you're playing on another system.

FaceBall 2000 - 16 Players.  I've only actually maxed out at, I think 11 people.  Really need to get 16 people together to actually play it.  Obviously, I used other Game Boys in addition to the Game Boy Player. :D

F1-Race - one of the few 4-player games - copies of this are usually super-cheap, so I have four. :D  Here's the trick though - the regular four-player adapter won't work on the Game Boy Player.  It has an original-style Game Boy ext. port (wide, similar to a USB plug) and there's no adapter that I've ever seen that lets you go from the wider-plug to the smaller plug (there is an adapter that goes the other way around).  I had to have one of my spare 4-Player adapters (also found super cheap) modified with a smaller plug on the end. :D

Super Mario Bros. DX vs mode - fun, fun. :D

Generic WaveBird - so, on the NES Classics titles, you can "clone" the game to another Game Boy Advance unit when you link them together.  For games like, say, Super Mario Bros., if you do this and play it in single-player mode, the second screen just shows what's on the first screen.  So, take the GBA unit of your choice, plug a wireless adapter into it.  Take you Game Boy Player, plug a second wireless adapter into it.  Boot up Mario in either unit, link 'em, then sit back on your couch and play Mario on the big screen, using your GBA to control the action. :D

One of my favorite things about the Game Boy Player is that you only need the disc when you first boot the unit - after that, you never need it again.  This has allowed me to pick up a few Game Boy Players without the disc - saving me some money to run these monster things. :D

I mentioned this before about Resident Evil Revelations, but what if you could stream games from the 3DS onto the Wii U and play them on the big screen in a similar manner that the Gamecube player played GBA games? The same goes for DSiware and 3DS Virtual Console games. This might justify the $50 price tag for Resident Evil Revelations.

I only want to do this if it supports my 3D TV.