Sprint's 3G network is presently so overburdened in many places, that it may as well be throttled. The fact is, if you load all your passive entertainment over WiFi (the same way it's done on an iPod Touch or WiFi only iPad/Tablet, which seem to sell just fine despite having no cellular connection), you can still have your TV shows, music and movies without putting the burden on the cell towers.
As for everything else you'd want to use mobile internet for, well, it certainly makes the browsing/Facebook/photo sharing/Twitter/email experience a whole lot more pleasant when you've got some good speed on your network. All things being equal, sure, Boost would be the better deal - but Sprint's native coverage is sparse, Sprint's 3G network is very slow, Boost's (official) phone selection is very limited and there's no way in heck anyone is using an iPhone on Boost.
I'm not saying anyone should switch if they're happy with Boost. If you have Boost and can stream Netflix without it stopping every five seconds to buffer, I envy you. But if you're considering making the jump from postpaid, would you really rather use a phone from Sprint's last year's reject pile on a tiny, slow network, or for about the same price get a used Atrix or iPhone 4 on AT&T with the only catch being "don't go over 2gb/mo"? How's the decision look, now?