I bought a new late 2013 model iMac and it has mountain lion installed. Now im wondering if i should upgrade I've heard reviews were it is laggy and disappointing and i don't want to slow my new imac down which runs fast now.
Here is a useful article that details some of the changes in Mavericks. In general, unless you have some odd piece of hardware that simply isn't compatible with the new version of OS X for some reason, it is probably a good idea to upgrade.