Just a couple of data points here. Applies only to Verizon. (I have enough trouble keeping track of my own carrier's efforts to squeeze more money out of me every month; I can't keep track of the plans of other carriers.)
() I have an unlimited (grandfathered) 3G plan on my Android smartphone. Throttling won't affect it since I literally never use more than about a gig of data services on the phone now that I have the iPad and (as noted below) I can also use a 4G hotspot+wifi for the phone if I care to do so.
() I have a separate 4G/LTE hotspot modem from Verizon which I purchased to replace a 3G equipped netbook. I use the hotspot for that netbook, my laptop, my phone if I bother to use it, and my iPad. Price is $50 per month on a two year contract for five gigabytes. Additional use if I ever need it (haven't needed it) is $10 per gig for a particular month. I believe, though I'm not sure, that I could get "unlimited" 4G/LTE service on that line for $80 per month.
The speed is phenomenal in 4G service areas. Up to about 20mg/p/s, close to what I get from my cable modem at home and indistinguishable from eifi on that service on the iPad. When I exit 4G/LTE service areas, the modem adjusts automatically to 3G service, sometimes at 1/10 the speed. Not nearly that perceptible as a user, but definitely noticeable. As others noted, there is no standard definition of what constitutes "4G" service. Verizon's service is much, much faster (and more widespread) than other carriers though AT&T is trying desperately to catch up on both counts.
Battery life for the mobile hotspot is about 4-5 hours in a 4G service area. About an hour longer when 4G service is not available. Not as good as the iPad's battery life but I'm seldom away from a charger for more than five hours. Haven't had the need to buy a separate charger for the modem.
Verizon offers (legal) tethering capabilities on some (all?) of their 4G smartphones. It's an additional $20 per month and would enable sharing of the phone's data plan (same $50/5gig deal) with the iPad. Doesn't make sense for me given my anniversary dates for various devices means I would have to cancel my mobile hotspot contract and pay a penalty. It's also the case that the 4G smartphones tethered speeds are substantially slower than the mobile hotspot (though still much faster than 3G.)
All in all, the major carriers are facing major bottlenecks from increased demand on their 3G networks. Thus the push to move users to 4G, to throttle heavy 3G users, and/or to shift costs from light to heavy users.