Rockauto has been great to me, I buy 90%+ of my parts from them. I've had pretty good luck with their cheap stuff (SKP, Ultra Power & others), but I try not to go too cheap on hard-to-replace stuff or crucial components. No sense saving $20 on a timing belt job when your engine is on the line if something breaks. I've bought many a SKP & Ultra-Power sensor and nothing has died on me yet. I have a feeling they come from the same factory as many of the (formerly quality) name brands that are also outsourced overseas, just without paying for the name.
Ultra-Power can be a good gamble sometimes, I've received a lot of stuff that's just reboxed OEM or top-tier brand. Other times it's foreign made, arguably junk, though again I haven't had any real problems with their stuff.
I'm a big fan of their closeout inventory. My older vehicles, that's pretty much all they get (when it's available.) I can see some trepidation buying rubber parts that might degrade (belts for example,) but what can go wrong with a spark plug or sensor that's been sitting on the shelf for a few years? I'm game for buying anything closeout, I've bought it all-- filters, belts, shocks, bushings, brakes, etc. If the part looks suspect, I toss it aside and eat the couple dollars. I've saved a boatload of money on closeout parts, it certainly helps me keep my 200k+ mile vehicles on the road.
As mentioned earlier, there's always a 5% off coupon to be had. Just search on Google, you'll find plenty of them. Retailmenot.com always has them.