For the average person trying to improve their photography, a workshop can be a great way to learn. At the same time, all workshops are not created equal. I'll attempt to share my practical experience in attending them to help you choose the right one for you.
First and foremost, it's important to distinguish whether you’re going to be attending a photography workshop, or a photography tour. They both have the word photography in them, but the similarity ends shortly after that. A photography workshop would be comprised of students and instructors. A good ratio would be one instructor for every five to six students. The instructors would be able to provide you advice on composition, lensing, using filters and polarizers, aperture, shutter and ISO settings, etc. Bear in mind, they may not be familiar with your particular camera. As boring as it may be to sit down and read the camera manual, it pays huge dividends when on a workshop to know how to control the various features your camera may have. One important note; the smaller the class size, the less you'll be jockeying for the same location once you arrive at your shooting location
A photography tour on the other hand is really just that. You bring your camera and they take you to locations to shoot. For someone who is pretty proficient with their camera, but unfamiliar with an area perhaps they're travelling to, that might be just fine. But there's little chance you'll be able to get any instruction on improving your photography.
While on the subject of instruction, how does one go about finding a reputable workshop? If you do a web search for photography workshops, all sorts of search results will come back, but how will you know what to look for after that? I strongly suggest searching in a different manner, by perhaps looking on a photography magazine website and then look for available workshops within them. Find a workshop that looks interesting to you, but then give them a call. Find out what the teacher/student ratio is. How long have they been conducting workshops? Are they patient with all types of experience levels? And what costs are included in the fee? There are many fine photographers out there that lead workshops, but may not be the best teachers. And of course, if you know people who've attended workshops before, find out which one they got the best instruction or enjoyed the most.
Another thing about workshops is the cost. They can cost anywhere from a nice lunch out to a second mortgage and I've found there's not necessarily a correlation between cost and quality. I once had a "workshop" with a photographer in Hawaii who drove me around to great locations, gave great advice and was as pleasant as could be. I was his only client that day and it only cost me $175. Nice.
If you decide to splurge on a workshop, make sure you let the workshop instructors know what you want to get out of it. Say you want to get better at composition, or better travel pictures or whatever it is that you want to get better at, but let them know. Then they can focus their time with you on accomplishing your goals.
The workshop will probably also provide a list of "suggested equipment." You may only have a DSLR with the kit lens, and that might be fine. But there are also resources like Borrowlenses.com where you can rent equipment to fill out your camera bag for that trip. Find out from the workshop what's "most valuable" to have on the particular workshop.
My final words about workshops are to remember to have fun. If you're on a trip to a national park, don't forget to take your eye out of the viewfinder now and again and just enjoy the scenic grandeur. It's easy to get frustrated by having spent the money to attend the workshop and then feeling like you have to "get the shot." Just take the scene in, take a deep breath and return to the basics. Great photos will follow.