I started practicing with a set about 6 months ago. I found out a friend had a pair and asked if I could see them. She ended up lending them to me indefinitely, along with the appropriate Demura book. I too am using them with TKD and have just incorporated them into my open-hand forms. I found that it worked well with Dan-Gun, and with some modification was able to get lots of flipping blocks and strikes, as well as some backfists (which are excellent with the sai.)
I went to a competition (my first one) and used the Dan-Gun form. I realized that at competitions it's all about being flashy - not having good technique and an effective form. Control is important, but flash is more impressive.
However, I did run into a TKD black belt who was spinning the sais. He showed me how to flip them. I then had two karate black belts come over and tell me that was an inappropriate use of the weapon. The TKD BB told me later that he actually learned the spinning as part of a form. I figure spinning the sais is fun and cool-looking, and it's great exercise for my wrists - so I'm still practicing it. I figure if I ever come up against a katana-wielding opponnent, not spinning my sais will be the least of my worries. [IMG]http://www.fightingarts.com/forums/ubb/biggrin.gif[/IMG]
I found the flipping motion rather easy to learn, but I've found that it comes much more naturally to me now after six months of practice. It's also a weapon I can bring to the office and practice without scaring people (no blade or swinging bits.) If I'm sitting in my chair flipping one, the usual response is, "Ooh, can I see that?" Granted, I have a pretty laid-back office. [IMG]http://www.fightingarts.com/forums/ubb/smile.gif[/IMG]
Anyway, I like the sai. ATM, it's my favorite weapon - though I've only studied the bo, nunchaku and sai.