Firstly, it's a struggle to get into corners with them and if you do manage, then they're not very effective at removing spiders nests etc.. The over extended stock makes this even worse, I have a complex of barn conversions with approx 2" deep frames and the Xtreme can't even get into these corners at all, the stock prevents it happening.
Then, you have jet placement. On the sill version it's correct but the none sill, for whatever reason, the jets are too close to the center. A minor fault you might think but it shouldn't be happening at this cost and from such a experienced company IMO.
Then you have the wear rate. This I suppose is the trade off for weight but these things literally wear out in weeks. My medium tapertec was past it's sell by date after 3/4 weeks. It started off ok but the bristles began to bend inwards, rather than the usual outwards, which renders it useless even quicker than normal as it exacerbates the inability to get into corners even more.
They also really bang into the frames, which isn't good. The sill version IMO is just a joke, it completely ruins the brush and seriously hinders your view of the rest of the brush. Two skinny rows of skinny bristles aren't really going to clean a sill effectively anyway, you're just kidding yourself
But unfortunately, this seems to be where the focus is being directed- a swivel is far more effective and you can use a normal, better brush to boot.
Then there's the jet capsules, great in theory but how many people really change jets in the same brush? I don't and I don't know of anyone else personally who does. most people tend to pick one favorite, either fans or pencil. In order to accommodate the capsules far more bristles have to be removed- something that an Xtreme can little afford, if at all. You look through the brush in use to a light background like a skylight and you'll see considerable gaps where there's barely anything touching the glass. This makes the brush less efficient, it's lighter but less efficient. It's less efficient with harder cleaning too, like bird poo etc. and even worse on cobwebs that have hit the glass (take a closer, second look and you'll see)- sometimes taking some considerable time and effort to move on. All this begins to eat into the benefits of it's lighter weight and the balance, or trade off becomes narrower. Add to that, the fact that you have to buy a new set for every brush purchase so end up with a van full of them that you'll never use.
This might sound like a real moan, and you'd have a point but..... I think a lot of these issues could be put right or shouldn't have happened to begin with as the tooling will have to be paid for so could be stuck with them for some time.
The current form of Xtreme isn't IMO as good as the early versions that had a stock more akin to the supreme and had thicker bristle clumps. There have been glimmers of hope like the natural/boars? hair inner Xtreme and the tapertec but the latter faltered too soon. It's kind of frustrating because I think a far better Xtreme could be made that would be more universally suitable, usable and have a longer 'usable' service life- instead the lure of the weight saving over the other options seems to compel us to accept what's on offer. You just can't beat a good, proper brush that has most of the desired qualities instead of heaps of brushes that do one or two things effectively and miss out on the rest.