I'm thinking about getting some. What brand is everyone using? What brand to stay away from?

There are (2) types; 1 has the butterfly valves(like a carburator) and the other 1 has a sliding gate.
I have been running the prototype gate-type cut-outs I bought from Doug Thorley many years ago.
My experience with the butterfly type is that they are more prone to leak, whereas the sliding gate design kinda cleans it self when activated and simply seals better..
There are a couple of disadvantages of the sliding gate design, they are more expensive and tend to be heavier so I recommend mounting a small support bracket.
In the end, I thoroughly enjoy mine, open when U want and throw the switch to close. Neat to pull upbeside an Escalade when his bass box is booming with rap, open the headers and let it it rock..

Just my $0.02...

Do both designs allow partial opening in use, ie just enough?

I have the Summit Racing butterfly type, they seem to be the best deal. The butterfly type are fragile and don't seal perfectly. They can be opened as much as you want, but once they are cracked open they seem to be @ max sound volume already

Next time I'll use the guillotine type, for the reasons mentioned above.

Thanks, and Interesting.

I have the Thorley gate type. It's nice to have a full length,quiet exhaust to drive around with and the added power and sound when wanted. It makes a big difference in the way the turbos spool as well as the overall power. I have full length 2 1/2" mandrel system with the long case Dynomax turbo mufflers. My cut-outs are 3, and they branch off of the 3 1/2" downpipes where they neck down to 2 1/2", I would do it again,

I've had both types on my cars. with Sinitro's summation. Both sets of the 'butterfly' type that I had leaked and had other problems. The Doug's guillotine types have always worked for me. The set on my Cuda today are probably 15 years old and have been through the ringer. They still work fine. BUT! I don't think they make them like that anymore.

Electric cut-outs have been mandatory on my Mopars for decades. At first they were for the KOS classes that required a drive and that the car be fully street legal. But then, I realized just how much FUN they are on the street. Car shows, tunnels, other supposed muscle cars, Harleys, Escalates with boom boxes, etc. Personally, I wouldn't have a muscle car without them.

Oh, and one more point; you don't have to debate about which exhaust system (mufflers or diameter) to run. The cut-outs relieve you of any concern about back pressure. On my Cuda, I have quiet exhaust. I can listen to the radio, have a conversation at speed, and don't grab the attention of every L.E. within ear shot. Unless I want to. And occasionally entering a car show, I do.

Electric cut-outs are way COOL! There simply is no down side. Well, maybe the money part a bit. But they are WORTH IT. Do it!

I've got a brand new set of Quick Time electric cutouts with y-pipes and remote control that I bought for my Dart. I was looking forward to using them as I know a guy who has a set on his Falcon and they are cool but I have decided to sell them as I need other parts for my car that are more of a priority.

I have Jegs brand. Left side leaked after 5 years. No complaints. Easy to install butterfly type.Cheap

I have doug's guillotine style cutouts on my 68 GTS for 4 summers with plenty of use. I have no complaints . They both still seal up without any maintenance. You can open them to any %

Yes..
The Thorley sliding gate type can be partially open, just cracked a notch or wide open.
One down side weakness of the Thorley product is the double throw/double pole toggle switch is sourced from China over time it falls apart.
Being an engineer, my solution was to source a higher-quality, US-made mil spec toggle switch.
As I posted previously I bought 1 of his 1st sets almost 20 years ago.
@ the Mopar Spring Fling about 20 years ago, Doug Thorley attended showing his Nova funny car in the swap meet area.
Doug was sitting behind a small card table with his 1st prototype, I knew instantly what it was.
Bought pair of his 1st production units, drove out to RiverSide and picked it up at the factory.

Just my $0.02.....