Depends on the sextant itself, the person using it and the conditions under which it is being used. A good sextant, used by someone with skills and familiar with that sextant and under good conditions (flat seas, with a clear horizon and a clear sky) can result in some good fixes.

The actual use of the sextant is only a small part of getting a fix. You need to be able to do that math, which means, IIRC, you need at least two publications: an almanac to tell you which celestial bodies you are looking it and how to correct your height observed for the bodies observed (http://celestaire.com/catalog/products/2009.html), and a table like H.O. 229 (http://celestaire.com/catalog/products/5902CE.html) or H.O. 249 (http://celestaire.com/catalog/products/5907CE.html). It's been more than 20 years since I actually did the math or obtained a fix, so my memory might be a little off.

If you are on land, using landmarks is almost always more natural and effective. Also, it can be pretty tough to get the right horizon on land. You would either need a sextant with an artificial horizon built in, and I recall hearing that such things are made, or guess at the level at which the horizon might be, which is probably going to reduce your accuracy.