Long post coming up (sorry :))
I got a 7-string soon after Ibanez first started making them - a nearly new white UV777 for an absolute bargain (the guy who bought it new obviously struggled with it & PX'd it within a week or so).
My thinking was that the 7-string would allow me, with the aid of a capo, to play along with the CDs I was enjoying in whatever key they'd tuned their guitars to. Of course, that's a lot harder than it sounds, so I soon abandonned the idea. But even playing it for my own stuff, I found it really limiting (which sounds counter-intuitive) - it was far too tempting to just grind away on the low B. Also, I found it hard to solo with: when playing a 6 string in E (or whatever you tune it to), I would automatically gravitate to the 12th fret of the top string to start a solo, but on a 7 that's 5 steps too high. If you go 12th fret on the B string, the string below is a different interval. But my hand didn't stretch as well if I started at the 7th fret.
Instead of actually thinking through these limitations (which were really limitations of my playing & theory knowledge rather than the guitar), I just stopped playing it & sold it after a year or so.
Fast forward 20 years, during which time I've tuned to Eb, D, C#, C, B, Bb, flirted with drop-C#, drop-Bb & come full circle back to Eb on my 6-strings :roll:
A friend had a battered old 7 for sale, I borrowed it, REALLY liked it & found myself on an immediate creative splurge. My theory knowledge must be a lot better because it felt like I was using the full range of it rather than just transposing 6-string riffs down to the bottom strings.
I didn't buy that 7, but instead got a newer RG1527 from someone on this forum. It's been a game changer for me: not only do I play it so much that I find it hard to adjust back to a 6 :lol: but Kieran's outstanding setup (D'Addario 10-59, tuned to std E-B) & the heavier picks I started to use made me realise what an imprecise loose setup I was playing on everything else. I'm currently testing various gauges so I can get my 6s in Eb & C# to feel as precise & solid as the 7s do.