I both lease and own. Per others the only real value in owning is the lack of commitment. I bought my 622 because at the time I was still under an agreement with the 811 and otherwise would have waited for 6 more months. In my case since the rest of the equipment I needed for HD was already installed with the leased 811 it was very easy to buy the 622, install it myself, and activate it. If you are planning to own HD equipment don't forget that you will also have to install the extra dish, LNB, switch, etc. like kb7oeb said, which I guess you would have to buy as well. So that alone might be your deciding factor for your first HD installation.
In my case my leased 811 is still the primary receiver on the account and there is no fee for that one - included with the programming package. With the owned 622 I pay one "Add'l (HD) Receiver Access Fee" of $6 per month plus the $5.98 "DVR Service fee" (which is waived at the HDPlatinum level). The $5 per month for the second SD tuner in the 622 is avoided by having the unit connected to a phone line. I also have a leased 311 in the system and that costs another $5 "Leased Receiver Fee".
What will your final configuration be? Do a cost comparison. You might be able to drop to just one or two receivers and save those fees...