The complaints I have Sierra apply to most of the local auctions, so it is hard to single them out for it:
1) Watch out for fees / buyers premium / taxes. They add up
2) Lines ... its a larger auction and things can go slowly
3) Rules ... not a lot of exceptions to be made, unlike smaller auctions. Don't expect them to treat you too special.
With those out of the way, I like Sierra, and I continue to bid and buy there. They have a consistently good selection, and I believe they recently acquired a contract with phoenix police... so I've seen lots of seized goods showing up. This means some interesting and high quality items can filter through Sierra. Along that line, expect all the cellphones and such to be locked... they were probably stolen / seized / shut down.
I HIGHLY recommend viewing items before hand. Staff is AWESOME about letting me check out gear / laptops / cell phones... so if gear doesn't work, I really just have myself to blame.
Many times I see items going for above what I see as their value - but again most auctions are like that. Only times i've gotten burned here is when:
1) I didn't check out gear before hand ( because I couldn't get to preview ) or
2) I wasn't paying close attention to my fees as the price escalated.
Staff on site is very friendly and helpful.
I bid entirely online, so not sure what the on-site experience is, or how they are to work with as a "seller".