It seems to me that a 50% chance of death is a bit ridiculous when it's combined with a random power. I mean, let's eliminate the chance of death. Picking a power and random is already a pretty nasty gamble with pretty much the same 50/50 odds (since there are just as many negative powers as positive ones, if you're pulling from a theoretically infinite list). So your odds of getting a power that doesn't make you miserable is already 50%.

Now, figuring out just how useful or powerful you might be is kind of hard, since there's no real guideline for that. But I'd say that, being generous, you're looking at another 50/50 shot to get a power powerful enough, or with a correct nature, to actually be useful or helpful to you.

So your chances are: 50% chance to die. 25% chance to have a horrible power. 12.5% chance to have a benign but useless power. 12.5% chance to gain a superpower.

Gamble if you want, but I don't like those odds.