Well, I'm quite fond of X Gear of the Y style stuff, so having the end result be a composite of several tables sounds nice. This is only improved by conflicting results, by the way.
As for the actual effect, not sure. The most obvious would be that it costs mana to apply a temporary effect. Ideally the duration would be something simple like "until the end of combat" or "until you leave the room," but lasting X turns could work also. I'd also be interested in avoiding a mana cost, like by having as many bad effects as good, and a cooldown to prevent spamming until I got a good effect.
As for specifics, I'm kind of torn. On the one hand, I'd like it if there was a mix of combat and non-combat effects, with non-combat presumably being "you find more gold" and similar as often as "+1 to search rolls" and the like. Then again, I'd like interesting combat effects as well. Anyway, the issue being that the whole would probably be a bit less powerful if the two were combined, since the odds that my nice combat effects would get clogged by useless noncombat ones or vice versa would be pretty high. Less of an issue if there's a ton of bad effects as well, obviously.
The other thing I'd considered, which I suspect isn't a very good idea, is that it could act like a mobile jewelry vendor. Stuff a bag of gold (or something similarly valuable) in, get permanent randomly generated accessory. Trouble is, this would likely be less interesting (though you'd have to roll a lot less) and convince me to shovel all my gold in there in hopes of getting better and better items. Plus, the vast majority of the stuff I'd get would be permanent garbage I'd never use, presumably.
On a slightly different tangent, I suppose it could also work more like a Horadric Cube, in that I shovel components in there and get something randomly generated, but built out of and related in some way to the reagents. So for instance, if I put a bag of gold and demon hand in there, I might get a gold ring studded with demon claw cut into little "gems," which depending on the rolls might grant extra lightning damage to my attacks, protect me from lightning, increase speed, render me weak to holy damage, etc. Except ideally with more than one (or two, or three, for that matter) effects per item.