Echometers are a neat tool that uses sound to determine (among other things) the depth of fluid downhole in a well. Similar to radar or seismic, an echometer sends a sound pulse down a well and measures how quickly that sound bounces back. Coupling this data with some math, we can estimate the depth of fluid in a well. It's a cheap, fast and neat way to answer the question, "How much fluid is in my well?" With that in mind, it's important to understand that echometers have limitations. The fluid level calculated by an echometer has some error in it. I've shot echometers on the same well, back to back, and received different fluid level calculations, sometimes off by 50 feet!
This is not an indictment on echometers, but rather my point is to say that you need to understand the limitations of this technology and only use it when appropriate. If you are simply trying to get an estimate of fluid level, or perhaps just trying to figure out if any fluid is downhole at all, then an echometer is a great tool for this application. However, if you are trying to determine your bottomhole pressure for a reserve analysis, then an echometer is simply not going to cut it. In this case you need a lot more accuracy than +/- 50 ft of fluid. If this is the case, then I'd recommend running bottomhole gauges in the well and performing a static pressure gradient, which will result in a much more accurate pressure measurement.
In short, echometers are a cheap and easy way to estimate your fluid, but it's important that you understand their accuracy and make sure sure you are using the appropriate tool for the job.