Overall, I love my 540...I'd have gone for an M5 if I had the scratch.
Value, greglepore is right on but I have noticed these cars priced all over the board. Seen low mile, 1-owner e39 M5s north of $30k to high mileage but presentable 540/6 at $2500.
Mine is a fantastic daily, I get 22mpg, 6-spd, rwd, lsd...makes for a fun ride to and from work
The PO, only owner, didn't understand the idea of changing oil so he popped the engine at 110k, so it's got a junkyard dawg in there with 90k on it. Runs hard, burns nothing, no leaks.
Most importantly, you can bang wrenches yourself on these cars. "E39 Source" on YouTube is a testament to this. Bunch of Polo toting rich kids with very thorough and helpful DIY videos.