If this game looks interesting to you, the founder's pack is definitely worth it, especially if you want to get into the beta and don't want to wait for a key.
If you want to play around with all the frames/classes, the bonus XP is nice to have and you get some in-game currency so you can buy a few more slots to store your frames. You won't have enough slots to play with all the frames if you don't shell out a few bucks anyways.
I've put quite a few hours into it, but haven't played much lately because I've pretty much maxed all the frames and the PvE content is limited as of right now. I'll probably get back into soon since their upcoming updates will be focused on more PvE content and quests.
Considering this is still in closed beta and many months away from a proper release, I actually like it quite a bit. The combat feels good and the frames offer enough differences in playstyle to warrant playing around with all of them. There's a good community and the developers actually listen to feedback from players, so I look forward to see how this game turns out.
Oh ya, and no XP wipes even after the game launches.
From their site:
I think I still have 2 invites left. If you want one, post your email. First come, first serve.