It's apparently not quite the deal you'd expect, and of course, it depends on Verizon Wireless and (the new) AT&T's bad plans.
I'm not sure that there is a real story or not. I don't expect miracles anyway. The big two have been working a little bit more to be balance plans with service. However, if you get great coverage where you are--watch out for fake coverage maps from any of the big 4--Sprint should be a good deal. Now that Nextel is all but dead and the 800 MHz frequency band is available, there is a huge chance that Sprint could be dominant in certain markets. There are Spark areas like Milpitas, CA where Sprint is using 800 MHz, 1900 MHz, and 2500 MHz combined.
Verizon has dual frequency bands with their XLTE but who knows what device you need to make it work right now. I've searched multiple times to see if my current mobile hotspot will take advantage of it but they're not telling apparently. I'd prefer enhanced throughput.
Update 2014.12.30: I've been a Sprint customer since September 2000. I've seen great service and weird, poor service. Lately, it's very good everywhere but home. While I'm feeling just more than neutral about Sprint, Verizon (my mobile hotspot) doesn't really make me feel better, and the stories about AT&T and T-Mobile keep me away. YMMV--Your mileage may vary. Do your research.
Update 2015.11.19: The deal is back, mostly. There are more conditions, but I suppose it's a good deal, if you happen to have great coverage where you live.
Speaking of which, Sprint also made a big announcement about 77 locations for their LTE Plus service, which seems to be what they were calling Spark last year. (They were also mentioning LTE Advanced but I'm not sure if they meant that LTE was more advanced than their 3G service or they've actually put the newer technology into place.) If everything is working well, you can get tremendous speed. In fact, they had the service in town for a while. I was seeing 38 Mbps, up from 1-2 Mbps and then, a month later, it was gone and the old service had been restored. (I believe that they took the equipment back to their city of Overland Park, Kansas to help themselves.)
It was quite a bit better than the 7 Mbps that I normally see on my Verizon mobile hotspot, and Verizon still hasn't announced where they have their enhanced two frequency service--only that it exists.