Well, I just bought a 545. Unfortunately, I haven't been able to use it yet, so can't tell you much about it.
I tried to use it last weekend and had some trouble with it. Got it started at the truck, although the first start took a bit of effort. Reved up fine and seemed good, so I shut it down and headed over to where the log that I was going to cut up was. Fired it up there maybe 5min later and it would start fine, but wouldn't rev at all. Every time I tried to give it a little gas, it would just die. Tried a bunch of times, tried letting it sit a bit first a few times, tried letting it idle for 5min or so to warm up first, no luck. Eventually gave up and just used the 455 that I had brought from work. Glad I brought a spare...
Overall, not really sure what was going on. I brought it back to the dealer and they got it running pretty well there. Apparently with the Autotune, the first time you use it, you need to start out by making a few solid cuts and it adjusts itself based on that. So they didn't want to run it too much without cutting anything there (they had just run out of wood!). So, hopefully it'll work better next time. I had a bunch of logs dropped off at my house a couple weeks ago, so have plenty to cut up, just haven't had time. Maybe I'll get to try it out quickly this afternoon. Otherwise, I'm out of town for the weekend, so it'll have to wait until sometime next week.
All that said, I'm certainly hoping and expecting it to be a great saw (wouldn't have bought it otherwise...). I'll let you know once I actually get to use it...