I bought said lance in January, to use in conjunction with a 3600psi / 18lpm pressure washer. I would not use it for cleaning soffits and gutters except on places you can guarantee that there won't be any water ingress into the roof space. At height these lances are beasts - I have the back belt (essential for higher work, say over 15 ft) and have carabiners on it rather than the plastic buckles.
Yes, it does clean soffits and gutters well, but no better than wfp + tfr (and there's no risk there); is it quicker - marginally. At height the bendy lance is very hard to control accurately, thus it's difficult to get in nooks and crannies. It gives a hell of a kick to it when you start it off too.
Where it does come into its own is exterior building cleaning (pressure washing). Apply tfr via a wfp + trolley and hose off. It's very quick and gives a great result, especially on smoother-rendered buildings. Mind you, Nat will talk to you about "soft-washing", which has its place too, imo.
Am I glad I bought the lance (the 24' one) - yes. Do I use it for cleaning the insides of gutters - no chance - it'll blow 'em off the wall (I bought the U-shaped gutter attachment too, but have yet to use it). Would I use it for cleaning the exteriors of gutters, fascias and soffits - no, not worth the risk. Would I use it for exterior walls - every time. Worth the outlay - yes, with the right machine. Is it quicker than wfp on g, f and s - marginally.
Be aware that the (larger?) lances come in 2 weights. I have the lighter weight one and it's very hard work on the shoulders. You'll have shoulders like cannonballs after a while and even a glass fibre wfp will feel like a feather.