I have explored using them. The deal breaker for us was that they are not fully integrated with marketo and you would have to do a manual upload after the event. To me, that is just as useless as having a piece of paper to check them in and then manually changing the status after the event.
For printing specifically, I think it's pretty seamless but they provide (and charge for) the equipment such as ipads and printer.
Thanks, we have someone that used Validar and heard similiar feedback. We're looking into Boomset and ExpoLogic to eventually integrate with Marketo. For now it's as simple as setting up an account with them and sending our list for it to print onsite.
At the Marketo Roadshow in DC they were using Validar. I'll have to research more on PFL.