Modern Marantz receivers (including the 5006) include Audyssey room equalization software, which often contributes substantially to the quality of what you hear, especially if your room hasn't been completely treated with absorbers, diffusers, etc. In other words, you should expect even better quality sound than your previous receiver provided. See the Audussey FAQ/101 for details. The instructions in the receiver's manual are woefully inadequate for getting the best results. http://www.avsforum.com/t/795421/official-audyssey-thread-faq-in-post-1/51750#post_21782993
If you can, purchase the receiver from an authorized reseller. Marantz warranties usually do not transfer if you don't. If you're buying it from an individual, be sure to get his original sales receipt, since that's what the warranty is tied to. Be sure you're allowed to return it to the seller for a full refund if you find anything wrong with it (excessive noise, dead channels, etc). Also, verify that all of the accessories are included, at least the calibration microphone. Replacements cost under $30, but can be a pain to get.
The 5006 is more than 2 years old, which limits the amount of warranty time left. You might want to consider comparing its features with those of current generation equipment to make sure it isn't missing something that's important to you. For example, the highest grade of Audyssey, XT32, is now available in competing receivers at non-refurbished prices under $1000 U.S.