I have multiple Wilson Electronics boosters. The name change to Weboost came with a price increase. You can still find the Wilson branded products on Amazon and other retailers. As stated they are not magic. With Verizon, I average 18db gain using the Sleek.
I also have a Wilson Mobile 4G capable of upto 5 devices. While advertised at 50db, I usually see 35db gain. Best gain only possible when your device is very close to the internal antenna. The Mobile 4G has a 5ft area from the internal antenna. Anything more than 1ft from the internal antenna will see signal gain decrease.
Not sure if Wilson Electronics/Weboost can accommodate the new Sprint Spark 4G. The older Sprint 4G should be ok.
Wilson prices fluctuate on Amazon. The first link I posted for the Sleek all 4G, 3G and 1x was $125 a few weeks ago. Seems to be $148 right now. Second link is the 5 device Mobile 4G. Expect to pay about $80 extra for the Weboost label.
Wilson General Purpose 4G on AmazonWilson Mobile 4G on Amazon