Would you ever buy an apartment complex in an area with a population under 3,500 people?  It's within 20 minutes from a town with over 100,000.  

Sure. My offer got turned down on one just like that two weeks ago; look at the whole picture. Typically I would want a higher return, attractive neighborhood, maybe an opportunity to force appreciation, etc.

@Matthew Poff - probably not the answer you want to hear, but "it depends". It is a town that looks dead, or is it a quaint town with a revitalized main street? Do most of the folks already commute? What the general demographics in the region like job growth or increasing/decreasing population? 

If there is an oversupply of decent housing in the town with 100k population, then counting on folks to commute for 20 minutes will require a big draw like nature, cheaper housing, nicer housing etc..

So short answer is, I wouldn't rule it out, but would really dig in for more details.