I had a BS in Chem/Bio (intending to go to medical school) before going to Nursing school and getting my BSN. My first BS was way harder than the BSN, but I was also less interested in nursing than I was in bio so that could be part of it.
The main reasons I went to nursing school in the first place were family health issues and a general lack of confidence, but those things are gone and I'm glad to say that I'm (finally) starting med school in the fall. Based solely on my own experience I can only recommend that if you finish nursing school you take a school break and work for a couple years, make some money and have some fun. The good thing about nursing is that it's pretty easy to make pediatrician-type money if you are willing to pick up a little overtime, and you also have the flexibility to swap shifts with people and have two weeks off without missing more than a day or two of work. To be clear, I'm not suggesting you do it for the money - what I'm saying is that it will be a good chance to save, play and enjoy life a little while getting some patient-care time.
More to the point of the thread, I haven't actually started med school yet but I feel good about finally deciding to go. There are at least two docs I know of where I work (a surgeon and an anesthesiologist) that were nurses for years (as in 10-15) before they went to med school, so it's certainly doable, and I've never felt like anyone I interviewed with held it against me. Come to think of it, it wasn't really brought up other than as part of a 'why med school, why now?' type question, but no matter what you've done they are going to ask you that. I worked the answer to that into my personal statement, saying that after giving it a shot nursing left me unfulfilled and in a position sort of sitting on the sideline constantly seeing others doing what I wanted to. The only people who ever really talked crap or hated on me were some of my nursing classmates (particularly those planning on CRNA school) who found out through a friend what I was planning. Insecure people will resent you for trying to excel anywhere though, nursing school or not, so keeping your plans to yourself is something to consider.
If you work in some varied environments, say the floor, OR and ICU there is certainly something to be said for well-roundedness, but as with anything else it depends on what you personally get out of it and what you can verbalize to those who will be asking you about it. Nursing might not directly make you a better doctor but it will give you a perspective of health care that some others will lack. You'll also have the opportunity to work with plenty of docs you love and a handful you absolutely hate and want to be nothing like, and both are worthwhile experiences.
Anyway, this post is getting way too long but if you want to go to med school you might as well try.