Sure @louisdi. As with many things in the YMMV category that either depend on your location or some other concern, YMMV.
However, my answer is a resounding: Yes, the Fi blending is dramatically better than both RW-Sprint (and especially) RW-TM. The only caveat to that is that there are still certain cell-barren spots, where neither of the RW options nor Fi give us any signal. Mind you, I’m speaking from the perspective of having a family with 2 MotoG4-RW-3.0 (originally T-Mobile, lately Sprint) handsets and 2 X4-Fi handsets, and formerly having had 2 X1-RW2.0-Sprint handsets (both X1’s completely died last month, BTW).
But here’s a case-in-point. At home and in most places where I otherwise normally travel, RW/Fi-Sprint has the clear upper-hand in signal strength and quality. However, at my main place of work, it just so happens that the RW/Fi-TM signal has the clear upper-hand in both signal strength (and especially) quality. This signal difference between home and work, though, makes the Fi plan a killer feature. Our X4-Fi phones naturally switch to the stronger signal without me/us having to think about it.
Please parenthetically note that I can’t really speak for how it compares to pure Sprint or pure T-Mobile in my area, since I haven’t gone down either route. I only know a few Sprint and T-Mobile users in my area, who only use Sprint or T-Mobile due to family-plan reasons – not because they like the local coverage.