The 18 month commitment gives you cheap service calls ($29 vs $99) and a free install (vs $50 activation). Definitely not necessary though!
Either way, the receivers are leased -- that 625 will be free but the 942 usually requires a $250 payment to lease. Basically, with a 625 and a 942, you'll pay $5 for the extra receiver and $5 DVR fee for each receiver ($15 over your programming price). The DVR fees are waived when you have "America's Everything Pack".. As well, you get charged $5 per dual tuner receiver if they're not plugged into phone lines.
As far as the "leasing" goes -- you'll get new receivers (if you go directly through dish anyway), but when you cancel those go back to dish (keep the boxes they come in, they won't ask for the cables that came with them or even the remotes most of the time, they just want their receivers back hehe).
There's 2 nice things to note about Dish Net I've liked: 1. The additional TV fees are only per receiver not per TV (dual tuner, dual output boxes save ya a little), and 2. No commitments for adding new boxes
If you buy them, they're yours and yeah.. I hate how DTV does that!
Sorry if this sounded all a little rough or something, but yeah.. if you have any more questions or whatever, feel free to ask!