DSLR Cameras: Is it worth picking up a second hand Canon 40D instead of a newer model such as the 550 or 600?

Canon and all major manufacturers divide their models into different classes the beginner models (Canon 3 digit series) prosumer models (Canon 2 digit series) pro models (single digit series) flagship (Canon 1 series) Naturally, different models have different features geared towards different users.  In general Canon puts very comparable sensors in cameras of the same vintage, so image quality is comparable across all classes (aside from APS/vs fullframe sensors).  But you do get more bang for higher end models, such as: Better viewfinder Better auto focus system Better build quality (the 40D is weather sealed whereas no 3 digit series body is) A longer lasting, quieter shutter Better manual controls (Canon prosumer models have a handy wheel that replaces the four way controller) I personally have had a 40D since 2008 and it's a great workhorse.  However if you buy used, make sure you buy under warrantee because any repairs will cost to a lot.  I had to replace the AF system on my body ad it cost $400. Do keep in mind that body technology evolves very quickly and a beginning model from today may outperform the 40D from 4 years ago in terms of sensor quality and AF performance.  If you're a quality hound I'd check out sample images of different bodies on dcresource or dpreview before buying. Lastly, if this is your first DSLR I'd say go with the beginning model.  You'll have a lot to learn, will be able to take great pictures and should have a lot of fun with it.  If/when you start to feel shortcomings of the beginner body then sell it and buy the higher end one.

T3i is best entry level camera with highest cost to benefits ratio. Start there, 40D is past unless someone gifts you or you get for free