Hi,
I went on novomix when I was first diagnosed, to get some idea of what amount of insulin I might need, then went through a honeymoon period which threw this out of the window. Once out ot the honeymoon period, I went onto long and short acting (in my case at that time levemir and novorapid).
I think using a basal-bolus treatment aims to give more flexibility, so you can have different foods and give insulin to match. It also means that you can eat when you want, even missing a meal without risk of going hypo (which you would be more likely to with novomix).
It really is what works for you - if you've got good control with novomix and it's suiting you, then stick with it. I still keep a supply of novomix (which I had left over when I changed insulins) to use in emergencies (i.e. if I run out of the long-acting insulin) - it doesn't happen very often, but when it does it's a real life-saver!
Jen