Reel mowers are very inexpensive, with most reel mowers costing around $100. They're also easy to maintain, only requiring the occasional greasing. If the blades become dull, a sharpening kit can be purchased for $15. We've had our reel mower for 3 years and have yet to sharpen the blades.
Besides being cheap to buy and maintain, reel mowers are also safer than powered mowers (75,000 people go to the ER every year for lawn mower related injuries) since powered mowers have a tendency to turn rocks into projectiles. They're also quieter, allowing you to mow your lawn any time you'd like. Finally, real mowers are better for the health of your grass. The blades of powered mowers, whether gas or electric, cut by chopping and shredding. This leaves the grass more vulnerable to moisture-loss. A reel mower snips the grass much like a pair of scissors. This cleaner cut keeps your lawn healthy and looking neat.
The only downside of reel mowers is that they are slightly more difficult to push than powered mowers. If your yard is larger than a half acre, a reel mower may not be for you (however, if you have more than a half acre, I'd recommend xeriscaping the majority of your yard since lawns are wasteful in the first place).