I have used the Outback S-Lite for 2 years now. Easy to use and is very accurate. Would not be without it.
It knows where you are and where you have been so it makes it very easy to go around obstacles (trees, ponds, structures, etc.) and pick back up on your line. All of my fields are irregular shaped due to creeks and tree lines. GPS makes things much easier.
Chemicals are far too expensive to waste on guesswork.