SMP's can be worth it or not; there are no standards for when application, what linkage means, how courses are organized, what advising is done, etc. For that matter, there is no standard definition of what an SMP and each program is run differently. Here are a few items that you need to find out about each program you are considering:
1) Is the SMP designed for career changers, GPA enhancers, etc?
2) What is their criteria for acceptance? Do they require MCAT?
3) Is the program under the auspices of a medical school? Is it run under a general University administration?
4) What is their success rate? How many students do they have?
5) Are classes just SMP students? Mixed with medical students? other postbaccs? Or just standard class offered to all campus students?
6) Is there dedicated advising?
7) What is the criteria for certificate/degree? Are you required to get minimum GPA for a committee letter? (read next item)
8) What is their success rate? How do they count that? Do they deny committee letter to those who dont make minimum GPA, thus reducing those who apply and increasing there success rate?
9) What schools do they have linkages mean? Guaranteed interview? Placement? What connections do they have?
I could go on, but you get the idea