I have been using WordPress for awhile with the K2 theme and I would say that these two together make a powerful combination that is easy to use. I have not had any real problems that I did not cause myself from experimenting. Of course everyone should BACK-UP their websites in case of problems.
The current version of WordPress is 2.3 and works great the K2 theme. I am currently using K2 revision 562. The most recent versions of K2 know give you that option to run WordPress widgets or K2 sidebar modules. Keeping up-to-date is easy with upgrade plugins. I use InstantUpgrade version 0.2 by Alex Günsche.
View a list of plugins I use to run this site or see it on an About page subpage.