I'll start off with the biggest downside.  If you have a yard with lots of trees that drop branches regularly, even if they're just small twigs, your reel mower is going to stop everytime you hit one. 

I love my reel mower, but it's not for everyone. Mine is a Scott's.  I've heard Brill makes a really good one too.

Push Reel Mowers: Pros and Cons

Here are some of the other Cons:
 
The mower doesn't cut when rolled backwards.

The blades don't go all the way to the edges because of the wheels, so you'll have to overlap rows while you're cutting.  For the same reason, you'll leave a bigger area that needs to be weedeated (weedate?) because it can't cut right next to a fence or flowerbed.











If your lawn is uneven, with lots of dips and bumps, a reel mower may not give you a great cut.

You can't let your grass get too tall (or weeds in your lawn get too tall) before you cut it because the reel mower may simply push tall grass (and weeds) over as it rolls over it.  It's so easy to use the push mower that it's not a big deal to run it over the lawn once a week, though.

The wheels on the mower push the grass down as you cut, so it's possible that thin strips of grass in the wheel path won't get cut, even when you overlap rows (depending on how fast that grass pops back up).

OK, if the list above is starting to scare you away, here are the Pros:

You'll never have to buy gas, oil, filters, or spark plugs again.

Gas mowers are major polluters.  I've seen reports that claim running a gas mower for one hour causes as much air pollution as driving a car 300 miles.

I've done no maintenance to my mower since I bought it, other than spraying the blades with WD-40.  I have a blade sharpening kit ($20), but I haven't used it yet because the mower is still cutting just fine.

The way this mower cuts is supposed to be much better for the grass.  It's like using scissors on your lawn, and gas mowers can essentially rip the top off the blades of grass.

We don't have enough mature trees for that to be a problem, but a hard piece of mulch will stop the mower also.  When this happens I just roll the blades backwards with my foot to let the mulch/stick/whatever fall out of the blades and then keep going.  If you had a yard full of debris this would get old very fast.

This could have been a problem
on the mower I ordered, but the levers that set your cutting height kept slipping off the "teeth", so I had to keep bending them until they got to a point where they would stay in position.

You can choose from adjustable cutting heights.  Mine has Low, Medium, and High, with 3 additional settings in each height.

Reel mowers are quiet, lightweight, and inexpensive.  I believe I paid around $120 for mine.  Plus, you get more exercise than you would with a gas mower, since you are the engine.

That's all the information I can think of that might be helpful to you.  Let me know if there's anything I left out.  I'm happy to answer any questions you have.