I do have a two truck, and it is a very good locomotive. It can pull 15-20 cars up a 150' long 4% grade without any problems (and the only change is has from factory is battery RCS). On wet rail it is probably 13-15 cars up that same grade. The Spectrum Series is a very good series, and Bachmann is improving if very well.
I have a pair of shays both RCS/battery powered with installs done by TOC. They have performed flawlessly for several years in our weekly ops sessions. Each probably gets 15-20 hours run time per year in these sessions.
As others have reported, they are very powerful locos and will readily handle 20 cars up a 3or 4% grade. When I visited TOC I personally saw a shay drag 28 cars up a 150 foot 4% grade.
A couple of words on well known negatives. The first run (late 1990s) had a severe problem with the plastic trucks crumbling and later versions of the first run also suffered from some problems with the trucks. The current run of the shay has metal trucks which resolve all the problems. If you consider a used very early shay, you probably should mentally factor in the cost of replacement trucks (about $130).
The only other running "problem" is the well documented lurching when descending a steep grade under load. The drive train has only a single cut worm making the lurch unavoidable though it does not seem to cause any problems to what is otherwise a very solid and robust drive. A few folks have had screws in the valve gear and driveline back out though I have never had that problem. I think if the driveline is lubed according to the manual and all screws checked (and retightened with loctite) there should be no problem.
In summary, i think the B'mann shay is a well detailed model as well as having a solid drive - the current version is a bargain at the street price well under $400.
I have one of the early second-series truck versions from around 1998, that I bought unseen from the Train Stop store in San Diego. It has been a prodigious performer since Day 1 in every respect, easily hauling twenty loggin skeletons and a sheridan van on level track [I don't have grades as I run Gauge 1 live-steam as well].
I only have track power, BTW, due to the expense of battery power over here in yUK for my forty-something other locos in various scales.
I'm waiting for my three-truck to arrive as we speak - a 'soundless version' - and anticipate putting Sierra sound in it. With Dave Goodson half a world away from my current location, although I was within fifty miles of him last week, I'll do it myself - getting somebody to do it for you here in yUK is almost the same price as buying the loco.
I have all the Bachmann Fn3 geared locos, and the Connie as well, and all are fine runners.