I have written some nice looking front-ends using Perl/Tk. It is certainly easy to use and very quick to develop. As far as how professional your GUI will look - well, that's really up to how much time you are willing to put into the design.
If you want to see one of my better works, you can find it here. This was a front end to a class project we did for a real-time oscilloscope. We used RTOS Linux component in conjunction with a Keithley DAQ card. The link above is just a sample GUI with dummy data. Please feel free to play around with it.
I must add one more thing. In my Tk experience, I've discovered that even though the code will work on any Perl/Tk enabled OS (btw, I tested this on Windows and RedHat), you MUST specify ALL parameters to EVERY widget in order to achieve the same look on different platforms. So, if that is a concern for you, please take this statement dear to your heart. I know for a fact that my program does not do a good enough job - I saw major differences b/w Windows and RedHat. I hope this helps.
