Are you planning a building a new site, then go straight into 2.5. Keep in mind this release is based on the same 'core' as 1.6, 1.7, so it has been tested since January 2011...and it is stable.
Do you currently have 1.5? Now is a good time to budget and plan for your upgrade. It is also a great time to do some 'website cleaning', maybe upgrade to new components, new template, etc...The upgrade isn't 'difficult', the time consuming part is the 'clean up' that you will want to do because you are upgrading...For example maybe one of your favorite components only exist in 1.5, so you will have to find a new one that works properly for 2.5, etc. Most tools that work in 1.6, 1.7 will probably work in 2.5, but you will have to test.
Not ready to move? You can 'stay put' if you must...but you will be 'forced in time', because I am already seeing many components only support 2.5 releases...I have to believe that in a few months...the components that you always loved, will only be supported for 2.5 releases...so if you are not ready now...at least begin your planning...so can can move when you really need to. Joomla.org will only support 1.5 until April 2012 (perhaps until September 2012)...that doesn't mean that your site will turn off...but moving forward will get more difficult the longer you wait.
I have been using 2.5 since december (my training course was done in 2.5), and I am finding it easy to work with...I am still learning more each day...and I am thrilled! A job well done by the Joomla team again!
In my course Getting Started with Joomla! I have a lesson dedicated to explaining about the different versions of joomla. Click here for a free video on the explaniations of the different versions of Joomla!