We chose an iPad 2 running the latest version of iOS as the input device. I have turned on guided access and set it where the only accessible section is the actual webpage portion of Safari. I had the login page of the time clock software pulled up before turning on guided access. I am not sure that our employees would want to be browsing Facebook at the time clock, but you never know. We mounted the iPad to the wall using this mount. The mount was very easy to put together and it seems very durable. One nice touch of the mount is that it enclosed the dock connector portion of the sync cable inside of it. This would keep someone from unplugging it (to play a joke).
We chose TimeClock Plus Web edition because it integrated nicely into our payroll system (I've been told it will take our payroll process down to 3 hours from 1.5 days). The software seems to work well enough. It's not iPad optimized, but we've had a good experience thus far. Their support has been top notch as well.
This is a perfect example of how flexible the iPad is as a device. It can really be whatever you want it to be. I'd venture to say that this solution cost about the same as buying an actual time clock as well.