My company is a member. I don't always get a chance to read the magazine...but I'd still be a member. They provide useful updates and they are very helpful if you have regulatory questions.
For example, I had a contract to prune trees on the grounds a large company who is very on top of their safety protocol. Their expectation was that we comply with OSHA regs (obviously), and they outlined standards for working from height. Everything they had insisted on full body harnesses (not a big problem) and being tied into certified tie in points, etc... Obviously these were provisions that won't work for climbing a tree. I called the OSHA guy at TCIA he ran me through the regs...(what that one amounted to was they were refering construction regulations which have no relevance to tree care). I could go back with confidence that we were in great shape. I did a lot of digging to figure that one out and was lost after trying to read all of the junk I could find online. I figured "what the heck, I pay TCIA dues, so I'm going to call. That took 10 minutes...I should have started with them!!
Bottom line: I think TCIA is well worth it.