It is indeed. Normally gets used with the regular length hitch and a 145 litre Really Useful Box for bulk shopping and similar - a combination about which I only have one bad thing to say: It won't fit through a standard width doorway (the Y-Small will).
Is the difference in width worth the awkwardness re doors?
Probably depends on what you're using it for and what your doors are like. For shopping, I bring the bike in first, then just lean the hitch arm on the doorstep
[1], carry the contents of the box inside, then the box, then lift the trailer, and carry it through sideways before dismantling and putting it inside the box.
If you've got a communal hallway or a door opening directly onto the street or something this could be annoying. OTOH, our London flat had an immediate 90 degree turn through a second door that didn't open fully, and no trailer (other than perhaps of the granny's shopping-trolley style) would have fitted through that.
I decided that if I wanted a trailer, versatility for large items was most important. In normal times, I manage the day-to-day shopping just fine with a couple of panniers. I've been using it a lot more since March, to minimise supermarket visits.
I want to get some wood, and don't fancy keep spending €20-30 per trip to the wood shop on a green wheels, if I can bring it on a trailer. But at €350 for the trailer, and another €95 for the extension arm, the cost benefit analysis is not clear.
Yeah. My original cost:benefit was primarily against the cost of supermarket deliveries, which made the whole proposition economically dubious, even though I got the trailer heavily discounted due to shop-soiled condition. If you can offset against some car hire or building materials deliveries, it starts to look better.
OTOH, the convenience for tip runs and awkward items has been brilliant.
Have you used the trailer with the Brompton? How does it integrate?
I've got a sturdy hybrid with low gears and an electric assist ICE trike available. Why would I want to use it with the Brompton? (I have used it to transport the folded Brompton a couple of times.) Similarly, I haven't tried with the Streetmachine (good at heavy loads, not ideal for stop-start traffic), though I'm sure that works fine.
The trailer bed is reasonably level when coupled to something with a full-sized rear wheel. With the 20" wheel on the back of the trike, it leans noticeably forward, and the Brompton would exacerbate that. But that's probably fine, unless the load is quite droopy - just means that any lights on the trailer are pointing upwards. Obviously the longer hitch means a shallower angle.
My main concern with using the Brompton to tow it would be the homeopathic braking. I know people do use more modest trailers to good effect with Bromptons for multi-modal touring. I think they tend to be the seatpost-hitch type.
Am I right in thinking the wheels pop off easily? I'm wondering if having cycled to the B&Q with a Dutch accent, could I pop the wheels off and place it in the shopping trolley next to the Brompton while I wander round picking up supplies?
Yep: Wheelchair-style hubs. Press the button in and the axle slides out of the bearings on the trailer frame. It does benefit from a cable tie between a pair of opposing spokes to stop the rubber cover that keeps the road gunk off the release mechanism pingfuckiting if you brush against something.
What's braking like?
With the regular hitch, and assuming you've loaded the trailer with a reasonable consideration for nose-weight, it's just phantom extra mass. Which is to say that on an DF bike with rim brakes in the wet, you don't want to pick up too much speed with a heavy load downhill, and that a tadpole trike with three disc brakes can cope with anything you might reasonably throw at it.
If you get the nose weight wrong, you find the back wheel of your cycle is far too eager to lift under braking, at which point either your MTB reflexes kick in and you come to some semblance of a controlled stop and hastily re-pack the trailer, or... [left as an exercise for the reader].
When I've used the long hitch, I've been sufficiently concerned about security of load that I've avoided picking up enough speed to really test the braking behaviour.
I have a pair of Ortlieb back roller classics, tho one had an adverse reaction to Tyler hill and no longer has the water proof qualities it used to have 
You can get repair patches of the various Ortlieb materials. Though I bet the damage is in an awkward place to patch. I have a front roller with a pinprick hole on the side from some sort of Sustrans jungle adventure, which patched admirably well. And another with road-rash on the bottom corner from a diesel spill incident which, while now rain-resistant, is best regarded as Carradice-quality waterproofing, and not to be placed on wet ground.
[1] Getting the nose-weight wrong while unpacking can be hazardous to your eggs.
