It was a fun experiment, but I'll admit they don't add as much as one would hope. The piezo can basically cover the full range of the magnetic pickups. The pickups are kinda like the piezo with much less dynamic range. The LaBella strings were built around the piezo and have a noticeably weak response on the D string. One big benefit on my 6 string is that the magnetic pickups added enough weight to balance it. Without them, the body is too light and would result in neck dive.
I think it depends. I had a Mouse 30 with mags only and my current D4 which Rob custom built for me with the mag and piezo (discreet outputs). The Mouse 30 was interesting, but not enough to keep. I didn't bond with the neck - the radius was too flat for me.
The D4 mag and piezo on first blush sound more similar than different. The mag is a slightly lower wind (i.e., not as hot) S4 pickup - a soapbar like the one above or the Mouse 30. Originally I thought I would blend the two. The mag brings a punch that the piezo does not have. There are times that the piezo even with tweaking the wooliness is a bit much. Switching to the mag sometimes makes it sit better in a mix (not the piezo is a problem - I don't want to give that impression). I like the mag on Latin-y tunes sometimes vs. the piezo.
I have used the two blended at times and it is its own unique sound. Depending on how you blend, to me it's almost like mixing a DI signal with a mic due to the differences in pickups.
String choice has a lot to do with it as spade2you points out. The various RAs I've owned over the years have sounded different with different strings.