Ah, the evil stop descender...
(Just kidding!)
I can't resist chiming in on this discussion. As a bit of a gear junkie, I've taken a few different descenders caving over the past couple of decades - the big U rack, big J rack with hyperbar, micro U rack with hyperbar, Kong Speleo, Petzl Stop, and Troll Allp. I might have even been spotted using a figure 8 or a Munter (Italian) hitch occasionally, but I don't make a habit of it.
Anyway, the descender I have used most over the years (by far) is the Petzl Stop. It's not perfect for all users in all situations, but then what is? I would not generally recommend it as the first descender for a novice. Any person using one for the first time needs to know that the handle is like a park brake in a car - NOT used for speed control. That's what your other hand is for, just like any normal descender. It you don't grip the rope before you grab the handle, you will end up doing what chaz mentioned in his post.
Actually, I saw the same thing happen years ago to a novice using a Kong Speleo (similar to a Stop). He did a knot pass (for practice) while rappelling down a cliff, then grabbed the handle to continue his rappel without first grabbing the rope below with his other hand. He sped down the last 10m/30ft of a rope and hit the ground, breaking his wrist while still trying to grab the rope below his descender. He was lucky.
As chaz stated, there are also double-action autostop descenders available which apply a brake if you either let go or squeeze too hard. SRTE and Anthron make two that spring to mind. I have borrowed and used an Anthron, but personally it didn't feel as comfortable to me as the Stop. I found that squeezing the handle half way to find the sweet spot took some getting used to. However, I have no doubt that this extra safety feature all but eliminates the risk of an out of control descent caused by "grab reflex". Despite my opinion the Anthron is a popular ascender - it's smaller, lighter, and at under US$100 much cheaper than the SRTE double stop.
For someone who mostly does longer drops (say 60m/200ft plus), I would suggest looking for something other than a Stop. A rack would be more suitable, particularly if is a J type or has a hyperbar - thereby allowing you to add or remove friction while on rappel. You can add some friction to a Stop by using a braking carabiner (I always use a Petzl Freino with my Stop), but I found it still doesn't have quite the same measure of control as a rack.
I also found that the action on earlier-version Stops was not as positive as it is on the current version - which probably led to the nickname Petzl "Slow". On my old one, I often had to tap the handle out to stop myself slowly sliding down the rope - but of course I would lock the descender off when sightseeing and snapping photos anyway. Just something to be aware of with those earlier Stops (the model that has the metal carabiner clip, rather than the newer plastic one.)
I will say that once you master the handle, the Stop is great for doing multiple drops and passing rebelays. It has a convenience and ease of handling that is hard to match. Plus it has less bulk and weight compared to most racks. The comments from filox1 about friction vs human bulk reminded me of something. In recent years, I seem to be (unintentionally) rappelling faster on my Stop that I used to. Perhaps I'm not as light as I used to be, or maybe I drag more stuff underground now. I especially notice this on newer ropes, or ropes less than 11mm (surprise, surprise.)
However, I don't think filox1 has anything to worry about when it comes to moving parts failing on a Stop. I'm not aware of any cases where that's happened, despite frequently being used in mud, grit, ice, and so forth. The lower stainless steel bobbin (connected to the handle) does rotate, but only about 30 degrees. This pinches the rope between the flattened top surface of the lower bobbin, and the round upper aluminum bobbin which is fixed to the backing plate. I strongly recommend filox1 borrows a Stop and tries it out - he might be pleasantly surprised! I'm always willing to lend one of mine to anyone who wants to play.
I tend mostly to teach first timers on a micro-rack or other U rack, often with a hyperbar. I introduce the Stop and larger J rack (if appropriate) once the basics are mastered. The Munter hitch, along with Prusik knots for personal ascending, are taught in the "backup techniques" category. I know many cavers descend on figure 8's, but they're not my choice for a few reasons. Firstly, the figure 8 was initially designed for descending a double rope - and SRT caving uses, by definition, a single rope. Secondly, it twists the rope. Thirdly, aluminum 8's deposit an awful lot of aluminum onto/into the rope. And lastly, I don't allow them to be used on my ropes. It's a personal thing.
For personal use, I have recently been rappelling more on my Troll Allp than my Stop. The Allp is also a bobbin-type descender, similar in size to a Stop, but it doesn't have a brake handle. It does however have a continuously variable friction adjustment, which is great for a smooth descent - short or long drop, light or heavy caver, big pack or not, one or two person load, etc. Unfortunately the Allp (also known as Alp with one "L") is expensive and a little hard to find, even in the UK where it's made.
I think the key thing here is that cavers should make informed choices about what fits their needs and their personal preferences - and periodically reassess those choices while keeping an open mind. True of many other things in caving and life in general, really.
Well, that's my 2 cents for tonight.
Cave Safe