If you're talking a 20" wheel, then the bike is too small for the average grown-up. If it's a 20" mountain bike frame, it may be too BIG for you. I'm 5'8" and I ride a 16" mountain bike frame, which is just a tad too small for me, but very comfortable. Most 20" frames I've seen were carrying guys much taller than I.
Like HumanCar said, learn about proper bike fitting and make sure the bike fits you. This doesn't have to be a drawn-out, crazy process. Just don't buy something that's going to make your knees hurt after one lap around the block.
Also, as Neil asked, what type of riding will you be doing? I prefer a mountain bike for ALL my riding, even long road trips, for which I just change the tires. For riding the Cape Cod Rail Trail or one of the other fine paths on the Cape, Martha's Vineyard, or Nantucket, many people feel more comfortable on a "hybrid" bike, sometimes called a "comfort bike," which is more upright and has thinner, slicker tires.
Either way, unless they were really good bikes to begin with, $149 sounds like a lot of money, especially considering it's Yard Sale season and you will likely soon see $300 bicycles out on people's lawns for $5. Better yet, wait until the sales are over and pick them up at the curb for $0!