Joomla 1.6 only has another 2 months until it's not supported anymore. This recent Joomla 1.6 update or the next might be the last you see of it.
Joomla 1.5 is supported through this time next year.
This means tons and tons of changes to make each time there's an upgrade and already looking at Joomla 1.7.
If you're really needing Joomla 1.6 for something that Joomla 1.5 just doesn't have, then it's alright to use. But, if you're not using those features, and Joomla 1.5 covers it, you might be better served waiting until Joomla offers an upgrade path for Joomla 1.8...released at the end of this year.
Don't use Joomla 1.6 just because you think it's the newest stable version...it's a work in progress and a different name for Joomla 1.8 beta. Joomla 1.8 is the next "long term" supported release after Joomla 1.5.
http://blog.lynda.com/2011/02/23/joomla-development-news-an-interview-with-jen-kramer-lynda-com-author/
Because of this different release schedule and if you want state of the art functions and designs, you'll need to work with the update timescale as Joomla has planned this whole year for a "development" phase for Joomla 1.8. It's not the same thing that you were used to with Joomla before.
nicely stated