It seems pretty good. I think the basic software is a spinoff/development of the Nascar Racing 2004 program (which yours truly shot some photography for, but...I digress). Setup requires some firewall configuration (if you have one) because the software on your machine has to communicate with their web/game servers. You don't "run" the program then do stuff - you load up their website and <span style="font-style: italic">it</span> runs the sim.
A few points of interest: my major beef with previous online games was that "anonymity" led to poor behavior in the game. In Race2007 (another good racing sim) you'd spend 30 minutes qualifying for a long race, only to have 2-3 racers line up across the track in a road-block. Sometimes they'd purposely bump, try to spin you off, etc. iRacing does <span style="font-style: italic">not</span> provide for anonymity. You play with your real, full name.
Another problem was always racers that just couldn't drive. Instead of practicing before a race, they'd just jump in and try to drive flat out. Non-intentional accidents happened all the time for no good reason. iRacing keeps track of your accidents, off-track excursions, loss of control incidents, etc. It balances those against your on-track performance and experience. Eventually you earn different "licenses" (by merit) that let you race in more advanced races.
They have a pretty wide selection of tracks available too. This is the first sim I've seen with <span style="font-style: italic">all</span> the Daytona tracks (oval, full road course, small cart track, and bike road course). I'm pretty familiar with Daytona and so far the modeling of the track is pretty darned convincing.
You can practice on any track you available, at any time, for as long as you want - but races are done on a scheduled basis, as are group practices and qualifying.
Performance seems to be pretty good too. I use a halfway decent homebrew PC, 2gb-ram, 2.4ghz quad-core processor, and a Nvidia 8800gtx graphics card. Running at 1280x1024 resolution I get very good graphics, smooth, and crisp.
Here's the downside: <span style="font-weight: bold">$</span> ! You <span style="font-style: italic">have to pay per month</span> - or buy blocks of months up to a year (at a discount). After that you have to start paying for different additional tracks and cars. They're not cheap either. The tracks and cars run $15-$25 (one time licensing fee). In the grand scheme of things it's relatively inexpensive compared to "real" racing, but there <span style="font-style: italic">is</span> that price factor.