I agree with pretty much everything bluesand said.
I'm just finishing up an MS in biology with an evo/eco type of focus and for me it has been completely worth it. I don't know of a single person in my cohort who wishes they had gone straight to PhD, and I do know several people who came in straight to PhD and wish they hadn't.
If you know exactly what you want to do as far as research goes, then you could still try to go straight to PhD, but there is no GRE score you can get that will make up for a low GPA and not much research background. I, my boyfriend, and my roommate all managed to make ourselves much more marketable to PhD programs by doing an MS first. In addition to having the time and experience to develop a much better idea of what I wanted to do for my PhD, gaining research experience and publications, and proving that I was capable of handling graduate coursework, I got lots of experience just talking to professors and other scientists. I'm much more confident talking about science with a wide range of people now and this served me very well for interviews.
The one thing to remember though, is that it's not the degree itself that helps your application, it's the experience you gain doing the degree, which varies widely depending on what you put into it. You won't get in to a better PhD program because you have an MS. You will get in because you got publications, research experience, and communication skills. If you don't make the effort to do those things you come out of your MS program no better off than when you went in (but down 2+ years of your life).
