Great post Azureal.
I find the MZ-N710 to be popular model on eBay, from that I infer it was a popular model with people. It's a middle-range unit with a nice feature set, only the plastic backshell and the single line display hints at cost-cutting. One can find these in excellent condition on eBay but if going that route always ask the seller the condition in their words and ensure there are clear, large photos of the unit and equipment.
Just to give my own opinion of models: I think the E510 is a good choice for a player unit, if like me you mainly use LP2 recordings then one can achieve 100+ hours of playback: http://www.minidisc.org/part_Sony_MZ-E510.html
The R900 is excellent in terms of looks, functions and ergonomics. With some searching on eBay it can be found in excellent condition with "low mileage" for not much money indeed. It's my favorite out of the whole range. It also has a full aluminium shell and feels, expensive and solid. Ironically, it can be very attention grabbing when everyone around you has an iPod of some kind.
Of course, flash-based DAP are extremely popular now. I found this Google Trends graph of the term "minidisc" interesting. Looks like a gradual (logarithmic?) downward trend since Google started tracking search terms in 2004.
http://www.google.com/trends?q=minidisc