Here it is:
I do TTW-customization on almost all of my sites!
Most of them are simple sites that use no custom logic and have only some templates and CSS files customized. I work with people on these projects that don't know Python or anything of the Zope/Plone stack. Explaining them the portal_skins ZMI is easy enough, explaining them shell access, server restarts and theme products is just not productive. And restarting the server for one change would take down the four other simple sites hosted on the same server as well - that's just not a good idea.
One thing I'm not going to do however is doing any kind of TTW development. My simple rule is to never use Python scripts in the customs folder, just customized page templates. For Python code and any kind of logic you need to know Python and some parts of the stack anyways, so the step to file system based code is a lot smaller and the advantages of proper development tools gets bigger.
The only thing I personally have found to be too cumbersome is to do Plone version upgrades with TTW templates. I need good diff tools to compare the changes made to the original template and my customized versions (say main_template), so I usually end up dumping the whole templates to a file system theme product before the upgrade and do the diffing and adjusting work there.