I had a Benavente seven-string for a while, and really enjoyed it. The playability was great, and I had very few issues...beyond my wedding ring hitting the C & F strings whenever I'd play lower strings in upper positions. (Boy, did that sentence suck...sorry about that.) For a quick minute I even considered an eight-string, mulling over the idea of a low F#, but decided against it. Aside from playability concerns - and the notion of how often I'd actually leverage that much range - one of the other things that held me back was strings. I couldn't find an 'F' that did anything for me, so my expectations of going lower than a 'B' were hindered as well.
Ultimately for me it just came down to too much instrument - weight, managing that neck, electronics, etc. I'm an avid six-string fan, but I no longer have the urge to go beyond that. Good luck with your hunt, and I'd definitely recommend talking with Brian @ The Low End about options.