Dave I've only used it in 4lb and I do like it. It's strangth is unbelievable. BUT I find it hard to manage due to its small diameter (and my fat fingers).
I think in future if i buy the brand again I will go to 8lb, not because I need the strength but because I'll find it easier to tie.
That's if I decide Fireline isn't good enough. Frankly, after trying a range of other lines, I'm still not convinced I need to spend more money. Maybe if after longer testing I discover that the other lines stay true for a substantially longer time.