I play mainly 6 string, I have a few fours and fives, I have a few 7's too, even a 10 and a 13 string bass. I personally find 6 the most useful, though I often find myself gravitating towards the 7 string for the additional highs, it's great to swap roles with the guitarist in a band while still playing a bass. $ strings is all well and good, but I need more, and 6s and 7s provide that. Useing various effect, reverbs and delays etc. you can produce some fantastic soundscapes. I deffinitely suggest people should at least try a 6 string, they reall open up the possibilites.