I use and recommend the single probe Rapidtest moisture meter. It is available from most of the major garden centers for under $20. This meter reads consistently without using batteries. It is invaluable for determining watering schedules, which vary tremendously from plant to plant, overwatered conditions, and uneven moisture distribution within the container. Rapidtest also sells a shorter, two probe model which should be avoided. Make this investment, monitor conditions regularly, and reap the rewards at harvest.
Growers that allow their medium to dry out to the point at which the leaves "droop" are reducing their final yields and quality. The medium contains a certain amount of salts that dramatically increase in concentration as the water dissipates. The roots can be repeatedly stressed going through this technique called "wet/dry cycle". The plant is being deprived of moisture that would be available to fuel additional growth and suffers.
The moisture meter's probe should be inserted to various depths to accurately assess conditions. The Rapidtest has a 1-4 scale on the meter, but what is imporatant is relative moisture. The lower potion of the medium in the container should not be so consistently and constantly moist as to "bury the needle" at the top of the scale. The middle depths of the container should be kept in the upper half section of the meter's range and the top should be allowed to dry out to the lower half of the range before rewatering.
This is far more accurate to the lifting and guessing game played by many. The weight of the container does not indicate where the moisture is inside. A grower would never really know if things at the bottom were oversaturated without a probe to tell them. If the bottom is soaked and never dries out, the container feels "heavy" even though other areas may be quite dry. Many grower use large, tight grids of 3-5 gallon containers which can amount to 40-100+ containers. How could they use the lift and guess method, if they can hardly reach some of my plants just to water and prune them. It would be impossible and bad for their backs to use anything except a moisture meter. In other situations the plants are attached to fixed supports, such as SCROG or simply tied up prohibiting movement. There is no more accurate or versatile way to determine your watering schedule.
Added by ~shabang~:
I am one of those that plays the "lifting and guessing game" and I will always recommend that you use your senses to judge and understand your plants rather than trusting in a $8 Wal-mart toy.
If a grower has 40-100+ large containers then they are likely experienced and likely growing mostly the same crop. When you know your plant and your system you don't use a moisture meter. No serious grower that I've met personally does.
Moisture meters are fine for beginners IF you use multiple sample points in each pot. It's too easy to hit a pocket of perlite or just rub the sensor the wrong way. I'd rather judge by visual and tactile response rather than entrusting a wavering needle on an inaccurate meter. You know that when you lift up your container, is it wet? or is it dry?