I've got a Twin Bonanzas (and several friends with Twin Bonanzas, we're all weirdos that stick together). I love, love, love my airplane. Mine is a 1954 B50, which means it has the "red headed step child" GO435s. My engines are much harder (though not impossible) to support than the 480s, which are actually not too bad. Unless you're a ham-handed hack, the geared engines will treat you well once you get the hang of them. The engines themselves are rather stout, it's the gear boxes that'll bite you if you don't mind them. Ham-handed pilots will thrash the gear boxes, which cause them to start to make metal and ship that metal back into the motor. If you take care of the nose cases, the rest of the engine will just keep on trucking. That said, you're not getting gear boxes (or nose cases as many tbone owners call them) overhauled for $1,200/side. More like $2500-3k, which still isn't bad.
Once you're into the TBones with 480s, you've got a small variation between the models. The C50 was the first with the 480, and it's got a low compression 275hp version. The D50-D50E have 295hp GO-480s. All of the GO models are equipped with PS5BD pressure carbs. The supercharged birds (E50, F50, H50, J50) have either GSO-480s (with PS7BD pressure carbs) or GSIO480s (with a screwy Simmonds fuel injection setup). The fuel injected supercharged birds are probably the hardest to support of the 480s, just because the fuel injection system is hard to get serviced. On the carb models, as long as the carbs have already been re-built with "red" seals, they're quite reliable and have automatic mixture control. If they carbs don't have red seals already, they are very expensive to have overhauled (like $3,500 per side expensive).
My airplane does around 155kts true on around 26gph. The models with 480s will go a little faster, closer to 160-165, on similar fuel flows. The supercharged birds, at altitude, are a little faster again, but on closer to 35-40gph. None of them are going more than about 175kts (and only the supercharged birds will do that much). That's a bit slower than a Baron on a bit more fuel, but remember you're flying around in a living room with more baggage space then you can fill.
I've got the 3x3 bench seating (3 place bench in front, 3 place bench in back). So when I'm only carrying 4 people, each person has an entire seat between them. It really is a baby King Air. Some later models (or modified earlier models) will have different seating configurations, including a couch in the back. Some models also have air stair doors.
Useful loads are usually close to 2,000lbs (mine is 1950, which gives me a ~1200lb payload with full fuel). Early models like mine carry 134 gallons of gas, which gives you 3-3.75 hours of endurance with a 1hr reserve. The D50 on have at least 180 gallons, and the supercharged birds go upwards of 240 gallons.
The airframe is very solid, with very few ADs against it (the 5yr wing bolt AD being the worst one). It's easy to work on, with fairly simple systems. If an A&P can work on a Bonanza or a Baron, he/she can work on a TBone (and will probably prefer it, as things are easier to access due to its size). The airplane flies wonderfully, very stable and honest. That said, it's not nimble. Mine lacks an autopilot, but with 3-axis trim and its stability, I get along just fine without.
Hangar space can be difficult; mine lives in an covered tie-down because it won't fit in a T-Hangar at my airport. Ground handling is also a bit trickier, as you can't move it by hand unless on a really flat surface and most "portable" powered tow bars won't do it either, you really need a tractor/tug setup.
Another big "issue" on the TBones is the control surfaces. They're magnesium, so make sure whatever specimen you're looking at has good ones on it, as you don't want to know what re-skinned ones cost.
For the most part, parts availability is not really an issue once you know where to look. Other than engine parts for my hard-to-support 435s, I've not had any issues getting airframe parts.
It does attract a lot of attention at other airports, so add 15 minutes to every fuel stop to chat with folks who want to see it.
