Our first DSLR, bought back in 2009 (with EF-S 17-85mm IS USM and 70-300mm IS USM) and has since been our workhorse camera for birds, macro and general photography.
As we were starting from scratch we weighed up Canon vs Nikon (the two real options for birds given the supertelephoto lens choices). Although the D300 was very good on paper, we preferred the ergonomics of the 50D and Canon was generally the more popular at the time.
It has since been used with a variety of lenses, flashes and accessories and performed very well. The size is just right for my hands, especially with the battery grip on, although probably a little big and heavy for Gill.
With a good burst rate of 6.3fps (and high throughput coupled with UDMA compact flash cards) and reliable autofocusing it has been a very good camera for bird photography, nicely balanced on the big lenses and a decent viewfinder. It also handles well for macro with 100mm lens and big flash setup.
Having the rear joystick and control dial is very useful for quickly push/pulling exposure compensation, switching autofocus point or varying aperture/shutter in Manual mode. I miss this a lot when using non-pro bodies like the 500D. The 3inch screen is also detailed and bright.
Being able to micro-adjust lens autofocus is a useful feature, although with our lenses they're either slow enough or lucky enough not to need it. There are a number of other customisable options in the menus, particularly the C1/C2 registered settings options. Being able to setup a general portrait configuration and a high speed action setting, then be able to switch at the twist of a dial is invaluable eg switch from a duck on a pond to a flock of geese flying past in less than a second. Another feature I really miss on the smaller models (and even on 1D models, surprisingly).
The major downside to the 50D is its sensor. Although it packs in an impressive and useful 15.1Mpixels (birds generally need to be cropped down a lot even with big lenses) it can be very noisy, particularly above ISO800, with heavy shadow noise, high ISO noise and banding. This is a shame because the body itself is just right for what we will be using it for but we will upgrade it sooner rather than later, for reduced noise more than anything else.
It's also just a little too old to include HD video modes. This isn't a big deal for us as we almost never use it on our 500D.
If you're starting out in birding, wildlife or just general photography a 2nd hand 50D would be a very good choice if you can live with the sensor noise. The last of the true xxD line (before the 7D and 60D came and complicated things) it's a shame the sensor limits it so much otherwise I think it would be one of the classic Canon cameras.