Hello!
I know it's a bit of a pain using the search function as there are so many comments that might not be relevant to you, but have a read of this thread that was created for this purpose. The trouble with what you are asking is that there are so many people for and against so many different workshops that you will get a completely biased view. For example, I have had some great experiences with workshops that my friends don't like and have had a terrible time with and some of their favourite workshops, I have had a terrible time with.
http://www.sau.com.au/forums/topic/228238-read-this-whos-the-best-tunerworkshopresprayerdetaileretc/
To sum it up, do the research (there is no shortcutting this), work out who you should be listening to and who you shouldn't and take some decisions from there.
Good luck!