I'm enrolled in a second bachelor's right now. I completed my first BS (Computer Science) with a 2.25 a few years ago, and I had already taken the basic pre-reqs with a 3.0 in comm. college, so I knew I had a long way to go. I retook the pre-reqs and then I had a choice to make--I hadnt taken the MCAT yet, so I couldnt apply to an SMP (plus I didnt want to burn out), attend a graduate MS program (which wouldnt help my BCMP as much, and I wasnt really that interested in doing a research thesis), or take more undergrad classes. Since I needed a way to fund my classes, and also because it allowed me to participate in dept research (at the ugad level) and be eligible for some university honors (Dean's List, Departmental Honors, et.) applying to do a second bachelors made more sense than just another year of DIY post-bacc.
This was the best option for me because I needed more than 1 year of post-bacc at the ugad level. If you have relatively good grades and and just starting out, just complete the pre-reqs and attend an SMP. I think a second bachelors is better than an MS, because the variety of courses in ugad is much better. Plus, you cover more topics with less depth (which I prefer rather than a few topics in great depth).
The negatives for a second bachelors are that you often have to take some classes that you dont want to take--Foreign Language, English, Social Sciences et. These are the fulfill university graduation requirements. Before setting off on a second bachelors, *thoroughly* research the amount of extraneous classes you will need to complete and determine whether or not it is too much. If you can complete the second degree at your first ugad institution it will help because you wil already have completed the requirements.
Beyond the University-Wide graduation requirements, you will also need to complete some departmental requirements. For instance, if you want to complete a microbio degree at my school you will have to take Microbial Diversity and Systematics, and a class in Mycology, which wouldnt really interest me. Research these requirements too. By completing post-bacc a ala carte you can avoid having to take a lot of these tedious classes.
Fortunately for me, I attend a school that has given me 'life experience' credit to fulfill a lot of their graduation requirements, and the Biology department has a Physiology concentration that allows me to take only interesting classes like Advanced Physio, Medical Micro, Immuno, Histology, et. The only 'bad' class I have to take is Animal Biology (Zoology) because its a pre-req for a lot of upper levels here.
Hope this helps.