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In truth i have not spent much time with the X4 but I did own a Windsor Sensor S12, which was basically a G1 more or less a manual adjustment X4 and that machine, while it had some nice features, was rather dissapointing in terms of performance. The shop i work at is a former sebo dealer, we have a few left over X4s and i've tried them and i'm not impressed, the Miele uprights clean circles around them. I've considered purchasing one but i just can't justify the cost when i feel it's carpet cleaning performance is so meh.
I do currently have an ice blue Felix 1 and an Onyx D4 both of those are much better performers