Ive been using WiFi Explorer around my home network for a couple of years now. It is a superb utility program with a refined UI design that can gather and display a wealth of WiFi network data and allow you to turn it into useful information. Now, I am just a dabbler, a moderately knowledgeable home user with a moderately complicated network, so I cant certify that it will meet all the needs of a true network guru, but as my knowledge has grown, this utility has continued to impress me. Its a great learning tool.
While I havent actually run into _any_ problems or bugs, IMHO its important to note that the developer periodically provides (free) updates (frequently, butnot too frequently). You dont get the unhappy feeling that the program has been left orphaned with whatever bugs it came with, or that your $ spent will become worthless at the next OS X version.
A usage tip: I just discovered the handy Annotaions and Device columns (these might have been added in an update, because I swear I never saw them before). Apparently some devices broadcast their assigned network name (e.g., "upstairs-airport-express") and some do not. My Apple devices generally do, but do _not_ for their guest networks. I used the Annotaions field to assign understandable device names to all of my networks, which really helps sort things out if each Airport device provides three network names (I currently have configured 2.4 GHz, 5 GHz, and guest networks with different names, and have three Airport devices broadcasting those network names. Now I can easily see at a glance how each device is performing.)