Just to follow up. I did apply the LG8 update to my Galaxy S III and spent half a day yesterday testing in Waco. It should be pretty representative of what the fully built out Sprint LTE network is like because Waco was deliberately built out as a Field Implementation Test area by Ericsson, the Sprint contractor there.
Overall, the results were very positive. I had 4G service almost everywhere I drove around the city and surrounding countryside. There were exceptions, mostly in rural areas outside of major highways, and in some downtown locations blocked by relatively large buildings. But I did try inside one building about four blocks from a Sprint LTE tower, and got snappy data performance.
So the LTE map on sprint.com is overly optimistic because it shows blanket coverage everywhere. But overall, my results were quite satisfying. If I lived in Waco with this phone and this carrier, I would be pleased.
BTW, I first tried to see what LTE service was like there before applying the L710VPLG8 OTA update, which optimizes the threshold signal level for connecting to LTE. But I got nothing even in strong coverage areas with fast LTE after the update. So anyone (like the OP on this rant thread) who did not try updated firmware would have a very wrong negative impression. (The EVO LTE model is supposed to be getting a similar update soon.)
This phone, and this network, are keepers. I just have to wait a bit to get LTE coverage in my hometown.
(I posted a fuller review with mapped sample results, signal strengths and speed tests at S4GRU.com, but it is in a sponsors-only forum there so I could compare sample sites to S4GRU's exclusive maps of Sprint towers.)