I had the Thursday to Sunday camping ticket last year. I have had the weekend camping ticket before and can say for sure that having the extra day is a HUGE benefit. Going on the Thursday means you can get the best camping spots and get settled into camping nicely. And lets not forget all the great stuff that was going on on the Thursday.
T in the Park has to be one the best events I have ever when to. The atmosphere is just amazing.
If anyone is thinking of going but is not sure because of the mud and what not, don't be put off. I managed to remain fairly clean throughout the whole event and if it rained I just used my towel to dry myself off. The weather did not make the event worse, to be honest, it made it more fun!