I was using T4.0 (the one that was running on java 1.4 as it was a limitation of our project at that time) and I found a lot of good ideas in Tapestry. It is true, that each version of Tapestry is actually a new framowork with it own codebase. The only thinks in common are ideas, but they are evolutioning too. For example, Hivemind IOC, which was used in T4 is similar in ideas to Tapestry-IOC which is used in T5, but Tapestry-IOC also contains a lots of improvements, like Guice style syntax, etc. It is also important to mension that Tapestry-IOC is official part of Tapestry, when Hivemind was standalone project (It was an issue for Howard to keep both frameworks in synch - at least, that was one of the main reasons as I remember). BTW, no matter if you like Tapestry itself or not, Tapestry-IOC is worth attension even as a separate element. It is fully independant maven artifact and can be used without any reference to Tapestry. And some aspects of Tapesty-IOC are unique (like configurations) and can't be found in other IOCs (Spring, Guice). So even if you already have your favorite framework for web, Tapestry still has something to offer, at least for widening your horizonts. The only problem I found with this "improved" documentation, is that I was not able to find IOC specific docs, as they were created fot T-5.0 or T-5.1, old organization of docs was more intuitive for me.
In a conclusion: If you like new ideas and enjoy discovering something new then you need at least to play with Tapestry. Maybe you will find out that it not what you need, but I am sure you will find it interesting anyway. Choosing Tapestry for your business is completely different question and this is a problem of all this thread: Jan blames Tapestry because of political reasons, other people hails it for it technical side.
P.S. Today, For RIA applications I would use GWT, not Tapestry or anything else. But for page based I will take a look on a Tapestry once again.