Hi, welcome to the forums, there are very few reviews of this set but you should be able to take the reviews of the PS8000 (which is more widely reviewed) as appropriate for this set as well, apart from the THX preset and a few other features. There's a review of the PS8000 here -
LG 50PS8000 THX Certified 50inch Plasma Review at AVForums.com as I say, you should be able to take the comments about the picture of the calibrated picture as completely relevant to the PS6000. I would say that it is one of the best sets for the money from reading.
Image retention is a issue which has largely been in the plasma market, it is possible on LCD TVs but is very rare. The overall issue of permanent burn in is largely over, there are very few real cases of this problem and this is basically where an image which has remained on screen for an extended period of time effectively 'sticks' to the screen so it is always there as a ghost image, image retention is a temporary form of this and does still exist, the Panasonics are very resistant, but the LGs and Samsuungs are more prone, you shouldn't be put off though. I have an LG plasma (form the 2008 range) and I use it largely for gaming, I have had several gaming sessions which have lasted 2 or 3 hours, even more, and as of yet I don't have any permanent damage. It's worst in the early hours of the sets life so you need to be more careful then but I would only say that it was an issue if you spent more than a few hours gaming at a time regularly (without breaks). There are several methods which can prevent it becomming bad, some of which are very easy to follow, such as taking breaks after an hour or two when gaming and switch to regular freeview for 10-20 minutes, any image retention should wash out in this time.
Hope that helped, if you have any further questions then do ask