I rarely use it, so I don't know if it's really worth getting or not. Though I did like it for Donkey Kong Country Tropical Freeze and it's certainly handy for those occasions when playing something multiplayer. I'll probably be pretty happy to have it when Smash Bros rolls around.
It's a great controller for what it's worth.
It's one of the best controllers ever, just fits in your hand perfectly. It takes a while to get used to the second stick being above the face buttons, but that's the same as the gamepad as well.
It's certainly been worth it to me, especially with the monster battery that's inside. I've had it for a year, charged it once or twice, it lasts forever (like 80 hours per charge). So if your'e playing long sessions, it's perfect, especially compared to the gamepad which needs to be charge in the middle of a long play session.. I played the first four or five hours of Zelda with the gamepad, and the battery died (from fully charged). I then switched to the Pro controller, and I completed the game (over multiple sessions) without charging it once.
Just so you know, if you only intend to use it for smash, you could wait and buy the Gamecube controller and adapter that they'll make for it instead. But I'm sure the pro controller will also work fine for that game.
For me it makes the console. Before I bought my PS4 it was by far my favorite controller compared to anything else, and I have not played a single game on my Wii U without it. The few times I tried playing with the gamepad it only made me think that if it wasn't for the Pro controller I would've probably regretted my console purchase. So ye, not only is it a great controller, but compared to the alternative it's pretty much a must-have in my book.
Honestly, it comes down to what sorts of games you play. If you are playing a bunch of multiplayer stuff in there, you're gonna want a Wiimote or two, the pad and maybe one of these. It opens up local coop if you don't have enough wiimotes and it gives you an alternate way to play if you don't wanna hold that fat ass screen all the time.
I say worth it.