I work for a (larger) water district. These smaller associations generally form when the cost to maintain a connection to a larger water district ("city water") is cost prohibitive, and there is a source of clean water close by.
The upside is that your water can, in general, be cheaper. Also, if you are against fluoridation, you are in luck. Your water won't be fluoridated. There are enforced water treatment standards for smaller associations just as there are for larger, so I wouldn't worry about water quality otherwise.
The downside is that there are less users to spread infrastructure replacement costs among. So, if that storage tank needs to be replaced, and it's $250,000 cost has to be spread out among the 100 association users instead of City Water spreading it out among, say, 200,000 users, you can see what will happen. You will get a much larger supplemental assessment bill! So, if this is something that concerns you, you may want to ask about the state of their water infrastructure. Do they have any planned storage or treatment plant upgrades, and how will the cost be defrayed among association members?