If done by someone who knows what he is doing for your particular display using signal generators, measurement equipment and service menu controls, a professional calibration WILL look better. Perhaps not for every scene of every show, but better. The user level controls only provide so much flexibility and even the most careful pre-shipment calibration of service menu controls by the manufacturer can't take into account the differences between source devices and viewing environments.
How much better largely depends upon how careful the display manufacturer was in setting up the display prior to shipment and how much your DVD player or set top TV box is futzing things up. Some display manufacturers for example even ship their expensive displays with exaggerated amounts of Red Push that can't really be corrected with user level adjustments.
Also, the tools you have available to do user level calibration are pretty limited except for your DVD player. You can't run the Avia DVD over your cable TV line for example. So as close as you can come to setting things up for your DVD player, you will still be stuck with making some eyeball approximations for your other sources. A professional calibrator will hook a signal generator up to your cable or satellite TV box for example to make adjustments for that source as well.
My recommendation, however is that you do an Avia or DVE style calibration for the DVD (and then transfer what you learn about what the image is supposed to look like to the settings for your other sources as well) and use the set that way for the first several hundred hours. First, this gives the initial aging of the phosphors a chance to slow down and second it will give you something to compare to so that if you DO spring for the bucks for a professional calibration you'll know what value you received. Whether you choose to wait or not, for heavens sake do SOME style of calibration right away so that you don't leave the levels cranked up so high that burn in starts happening.
Personally, I like the way my Fujitsu P50 turned out with only Avia calibration, but I still might treat myself to an ISF calibration at some point once I get tired of tweaking things myself.