22 inch frame mountain bike for someone who is 5'8" I'm 5'8" and had my mountain bike stolen in the fall (bummer!). Just found a really good deal on a 2015 Giant Revel but it's got a 22 inch frame. Would this be unsafe/unpleasant enough that I shouldn't get it? Hard to say no to a deal like this!
Bottom line:
If the choice is between buying a bike that is too large or waiting for the right sized bike then the best choice is to wait.

Much has been asked and answered on this topic on Bicycles Stack Exchange:
How can I tell if the size of a bike is right for me?
Choosing the right bike size
Riding wrong size frame  


  Would this be unsafe/unpleasant enough that I shouldn't get it? Hard to say no to a deal like this!


It's a lot like shoes.  

Let's say a size 8 shoe is the correct size for you.
There is a really great deal on an amazing shoe but all they have is size 10.  

Would you wear a shoe that's two sizes too big?

Can you wear a size 10?
Yes, you can. Anyone who's had to live with the wrong size shoe will be reluctant to do it again. If it was a matter of wearing a shoe two sizes too big or walking through glass barefoot then it might make sense to get the size 10 shoes.  

How "unsafe/unpleasant" would it be?
Only you can answer this question because you are the only one who can judge your unsafe / unpleasant level.

The Correct Size

There are good reasons to get the right size bike:
- Comfort
- Injury Avoidance
- Safety  

According to a bicycling.com article the ideal size mountain bike for someone 5'8" is 17" to 18" or "Medium"


22" is two sizes too big.

According to the linked article:  


  “Investing in a bike fit not only ensures you are going to get the right bike, but you’re also going to be set up on it perfectly, avoiding injury, pain, and discomfort, which will make cycling even more enjoyable than it already is,” Erickson says.


What ever you decide - always test ride a bike before buying.