I think you would be happier if you built your own. there are definitely better parts available than what is on the Tank Stick. That being said, you can't build a better one for that price, and it is half off the new price for the Tank Stick, so it is definitely a good deal (assuming it still works like new).
It is also a cheap way to get started, and you will be able to find out what you like about the layout and controls, and what you would change if you did decide to build your own.
I really never thought I would say this, but I think I would recommend buying it. If you decide to replace it later, you should be able to get your money back. I no a few people over at BYOAC who started with these, and were happy to learn with them. I am sure some were even satisfied with them and never upgraded.
We have an X-Arcade cab at school with one of these, and I think I am the only one there that even realizes that there are better parts out there. Everyone else seems perfectly happy with them. The first sticks and buttons on my MAME cab were from xarcade, and they worked fine. I like the Happ stuff I replaced them with better, but there is really no gameplay difference. Have fun!