I have used the 550xp at my job a fair bit.. It runs like a scalded dog and I'm quite impressed with it!
But admittedly, I have no intention of replacing my NE346 with one as long as parts remain available to keep it alive..same goes for my 357, and 372 XP-Dubya.. My summer truck is a '78 Chevy K10. I enjoy driving and wrenching on that truck much more than doing the same on our '06 Silverado. It can't be plugged in and fail an inspection for emissions. There is nothing on that truck that can fail short of the frame splitting that I would hesitate to dig into and fix. I like being able to fix my own stuff, especially when it can usually be fixed rather inexpensively relative to more modern vehicles. I have the same mentality about my saws.
On the brand thing, Husky, Stihl, Johnny-Red (black and red Huskies), and Dolmar all make very good professional products, and they make their "cheaper" products as well. I'm a Husky guy, but that doesn't mean that I think Stihl's products are garbage, not at all. If that were the case, they wouldn't be one of the top tier manufacturers of power equipment now would they. I just like characteristics about the Huskies over the Stihls, a simple matter of personal preference. Admittedly, most of those things are fairly insignificant to the function of the saws themselves in the sense of where the rubber meets the road.