The size of your lawn is the key factor in whether a zero-turn mower is efficient. If you have a small lawn, or a big lawn with steep slopes, zero-turn mowers are not very suitable, and they could even be dangerous. This type of riding mower can roll over on an angled slope of more than 10-15 degrees, and it's not very good on wet ground, as it can slip and slide. This type of heavy machine can even damage your lawn if the ground is too wet, so you really need to pay attention to the safety guidelines provided by the manufacturer.
However, if your lot is large and flat -- at least one acre or more -- having a zero-turn mower will save you a lot of time. You'll no longer have to get off the mower to use a weed whacker, and you won't have to use a push mower to get at those awkward areas under the trees. The zero-turn mower can do it all.
As long as you choose the right size cutting deck for your sort of lawn, you'll even be able to spin on the spot, if that's what you need. If your cutting deck -- the part of the mower that actually cuts the lawn -- is too wide, you won't be able to get around your flower beds, whereas if you get one that is too narrow you'll find yourself going backwards and forwards over the same piece of ground, which won't save you much time. If you're worried about running costs, you'll find that gas-powered machines can be quite fuel-efficient, compared to other riding mowers. The compressed natural gas and electric models coming onto the market are even more fuel-efficient; so, overall, zero-turn mowers can be a very efficient solution, if you have a big lawn.