Disclosure: I work for the parent company of Zions Direct. The following is my opinion only.
I bought several corporate bonds through the auction site about six months ago and it was pretty easy. You can bid either the price or yield and are e-mailed if you are out-bid. It reminds me of eBay back in the day when it was first getting started. There is one interface quirk that I can think of, but otherwise it really is that simple.
You can sign up for a free account, but FINRA will soon require bidders to have a Zions Direct brokerage account open before they can bid. One neat thing is that my existing checking account functions as the cash portion, so I just recently received my interest payments there, but you can also elect to have the normal brokerage cash account.
After watching the auctions for about a year as an investor, I can say that you will find some good deals on CDs and bonds, and some really crappy deals as well. Since the issuer can set the market-clearing bid, I have occasionally not bid because the minimum was at a premium. And make sure you read the term sheets carefully so you are aware if interest has accrued. I got hit by this on one of my bonds.
All in all, I like the platform and have been happy using it.