If you have the correct dishes, switches, and LNBs to receive HD off of your present setup, and you are not trading in a 921/942 to get the $99 lease deal, then buying is the clear better choice, IMHO.
You choice would be $299 for a leased 622 or $499 to buy one. If you pay $499 for one, then you are getting a piece of equipment that you could easily sell for $400-$450 at anytime. So your net loss is only $50-$100. Plus you have no programming commitment and your monthly charge will be lower (in some cases by only $1, but perhaps $6).
If you lease, you pay the entire $299, have no residual value, have an 18 month programming commitment (that will cost you $13.33 per month to break early), and pay a higher monthly fee.
So the $299 lease is a terrible deal as compared to a $499 purchase ... . HOWEVER if you have to purchase a new dish, a new switch, and pay for an installation, then that could swing the deal back in favor of the lease.
The $299 lease charge on a $499 product has got to be one of the world's worst deals for a lease. I've never seen another offer where the lease upfront charge was 60% of the purchase price. Of course, if you are getting $200+ of services and switches, then that considerably sweetens the deal for the lease.
And if you are trading in a leased 921 or 942 and getting the leased 622 plus the dish upgrades and installations for $99, then that's a very good deal.