Definitely worth the money and time. One of the best study resources out there.
If I were short on money and/or time, I'd buy some of the later AAMCs (8, 9, 10, 11) first.. but I'd probably buy the Self-Assessments before buying the remainder of the AAMC exams.
Not only is it valuable as AAMC material that you can get accustomed to, it's also a much better way to diagnose weaknesses than just taking a single full-length. They did a pretty good job in selecting which questions to include as it covers a significant portion of what you're expected to be able to work with. This means that you can take the SAs and then pore over the data to figure out where you need more study time. That's a HUGE advantage. People tend to focus on developing strengths, but it's more important on a test like the MCAT (which covers so much material) to recognize and eliminate your weaknesses.
I would also take them after content review (which should really be a minor portion of your studying) but before fully starting FLs. Then, spend the remainder of your studying using the information you collected to review and eliminate weaknesses in between taking full-lengths. As in, take the SAs, spend the next few days on weaknesses (this means practice problems as well), then take another FL. Keep a list of weaknesses identified either from the SAs, FLs or practice problems and keep crossing them off as you go. Once you feel you aren't deficient in any topic, you can begin developing strengths by picking topics you are confident you can master with limited time investment. Just don't do that from the get go.. because odds are.. the relatively few topics you are able to develop into major strengths won't show up on test day. You're better off bringing up your lowest possible score (i.e. a test full of weaknesses) than your highest possible score (i.e. a test full of strengths).. since your test will likely be more towards the former.
