Lots of negative comments about this from people looking at it from the employees side. Have you considered the impact on the employee if you just hire them full-time and it doesn't work out? They've quit their other full-time job (yes, we hire part timers who are at other companies while they're working there), potentially moved house, made other irrevocable decisions related to their new job with you. It's a disaster if you have to let them go in 3 to 6 months and it's very hard to get re-hired once you've gone through that - not only does it look bad on your resume but your self confidence is shot.
So a 30 day try-out IMO works extremely well for both employer and employee. Remember, you're trying out the company too during those 30 days.