I retrieved my manual mower from my parent's loft and mowed the yard last week.
It is quite a workout but kinda of fun. It is a lot quieter than our current mower. The cut is pretty good but every so often some blades of grass will pop up after a section of yard is mowed.
I also went over every place two times from two different angles.
Also if the grass is thick it could be hard to push the mower through it. If using a mulching mower, the blade cutting could be finer when compared to the gas mower.
It is nice though that you can get some good exercise and it is quiet so you can mow earlier in the morning or other times without disturbing your neighbors too much. Plus no pollution (except carbon dioxide I expel) and no gas being used! Woo Hoo!