I'll give a big thumbs-up to the gas burner, too.
I have the Go Systems one, but as far as I can tell the only difference between that and the offical one is the size of the knob, and the Trangia logo.
What you get is a remote-feed gas burner that clips into the Trangia windscreen where the spirit burner would normally sit. The hose exits downwards, through a hole in the base unit that's standard on modern Trangia sets. It then attaches to a standard screw-on self-sealing butane/propane cartridge in the usual way. It comes with a draw-string bag, which allows you to pack the burner up neatly inside the Trangia set without anything rattling around. There isn't room (at least in a 27) to pack both gas and spirit burners at the same time, though.
In operation, you have all the normal advantages of a gas stove: it's at full power instantly, easily controllable and makes a satisfying roaring noise at high levels. Boil times are, as you'd expect, substantially faster than with the spirit burner. The usual advantages of the Trangia still apply: it's extremely stable and wind-resistant, and all the heat goes where you want it. My so far brief experimentation with proper cooking suggests that it's a massive win when using the Trangia in frying mode, where the spirit burner's lack of fine control can be problematic.
Unlike many gas stoves, the Trangia burner includes a pre-heat loop. This means that, once the burner has been lit in the normal manner, you can invert the gas cartridge to allow liquid (rather than vapour) to flow to the burner. As the liquid is heated in the pre-heat loop, it boils and produces gas for the burner. The advantage here is that you're not relying on ambient heat to boil the liquid in the gas cartridge, so it will operate correctly at low temperatures, and preserve the butane:propane ratio of the gas. I've had success running mine off pure butane (which is a available in the much cheaper aerosol can style cartridges used for catering stoves) via an adaptor[1], though have yet to try it in cold conditions.
The disadvantage of running on liquid is that the control valve - primarily designed for vapour - becomes much more fiddly, with all the adjustment over a much smaller range of rotation. It also introduces several seconds of lag, as it takes time for the liquid already in the pre-heat tube to be boiled off after the valve is closed. Not a problem when boiling water, but you may prefer to run on vapour when simmering or frying.
While this solves most of the low temperature issues, the other disadvantages of LPG do of course apply: you're still reliant on bulky, often expensive or hard to find, cartridges and really need to carry two of them on any non-trivial trip, as it's hard to tell how full they are without weighing them. It adds weight over an equivalent meths setup, though if you're using a Trangia in the first place, this probably isn't a huge problem.
There's also a multifuel burner available for the Trangia. I have no experience of it myself, but it's quite expensive and reviews seem to be mixed. A stand-alone MSR stove would seem like better value for that sort of application. Anyone tried one?
[1] This is a tool for people who know what they're doing! It's entirely possible to spew liquid butane everywhere if you're careless, and you have to be aware of the structure of the feed tube inside the cartridge and how it relates to the notch orientation[2], else your stove may flare unexpectedly. They're available cheaply from the usual eBay dealers of quality (though I make no comment on what quality) Chinese tat.
[2] The tube is L-shaped, pointing towards the side of the cartridge in line with the notch on the coupling. This enables the cartridge to be positioned sideways in a specific orientation and vapour drawn off, as in catering stoves. With a remote-feed stove and an adaptor, you have to prevent the cartridge from rolling accidentally and keep the notch up for vapor, or down for liquid.
