
I use a Cateye Octicube for exactly the things that you seem to want a bike light for. It is £30, it has a flashing or permentantly on setting and lasts a ridiculously long time (I can't remember how long, but long enough for you to not really have to think about it). It is really bright, a lot brighter than I thought it would be when I got it. I just use it on my road bike when I know that it is going to be getting dark by the time I get home, but I am confident that if it ever for really dark then I would be visible to car drivers using it.
Its also quite light, compared to a lot of bike lights out there, it just clips on and off so is easy to use. I'd reccommend it.