If you are single or have an S/O on a similar schedule and do not have younger kids, I think it's great.
Trying to get things done on a 5x8 work week sucks. Post offices, doctors appointments, grocery shopping at non-rush times, ect are all a pain in the ass.
Having a full day extra a week off (or even two on a 3x12 schedule) is wonderful. Plus if you have a longer commute you just dropped your gas consumption by 20% a week.
Downsides are risk of burnout...but really that can be said for even an 8 hour job and a cut into your downtime to do things like work out, relax, spend times with S/O or kids.
I used to do 3x12's and that was very nice having 4 day "weekends". Another attractive option that some employers offer is a 7 on, 7 off. You work 7 10 hour days in a row and then have 7 off. Depending on your lifestyle that could be great too.