A short term for something like "satisfaction + frustration"

Imagine that you are waiting for something you really want to happen, like maybe buying a new gadget. Since you really want it, when you finally happen to get it you feel satisfied and happy. However, it may happen that the gadget is not that awesome, it's just... good. So along with the feeling of satisfaction you feel a little frustrated, something like "Wow, I finally got it, it's awesome! But hey, I was expecting it to be a little more awesome...".

Or, as another example, you are waiting for some event to happen, maybe your favorite band's concert. And when it's over you feel happy because it had finally happened, but at the same time you feel sad that it's gone.

Is there a word or a short phrase to express this mixed feeling?

I would call that "anticlimactic", a form of anticlimax, where the resulting event does not really live up to the expectations of that event.

What you describe are bittersweet experiences.

I would call it underwhelming.

I believe disappointment is very close to what fits the bill: a long-expected resolution arrives and it's not what you wanted.

From NOAD:

disappointment (n.) the feeling of sadness or displeasure caused by the nonfulfillment of one's hopes or expectations