It climbs better than anything else I've flown, including the old 20 series Lears. The 'rub' is the cruise performance.
What the chart doesn't really show is the degree to which Mmo decreases with altitude.
Unlike the older Lears, the wing will buffet immediately as you reach Mmo. So expect a realistic cruise in the neighborhood of .78 at <410.
As far as high altitude operations, I'm afraid you'll have to wait for someone else to answer that.(you might want to try the FRACTIONALS board...bunch of FLEX guys over there..) My personal experience is limited to 2,500msl and my books are in a CONEX container, somewhere between Savannah and Kuwait City.
As far as passenger comfort. ..keep in mind that this is coming from a guy that has spent the majority of his time in 20 and 30 series Lears..... I would rate the cabin at about a 6-7. It's a stand-up airplane but in my opinion, it's still a little narrow. The LAV is a huge improvement over the older models (it actually has doors instead of curtains!) but I'm 6'1" and have to hunch over to use the thing.
The operation that I flew for also operated a 601 and the principles didn't seem to complain when they had to ride in the 60 ( if that's any consolation). As with just about any 'mid-size', cabinet space and storage is still a 'bit' of an issue. I suppose, if you haven't done it already, you really need to get in one and crawl around to get a real idea of what you're in for.