The Toshiba Gigabeat is probably the best current target for Rockbox.
Perhaps its main flaw is that it's available far cheaper than an iPod,
resulting in lower pay for the salesman. ;-)

Used F40s commonly go for under $100 (on ebay.com), although they commonly
specify US-only bids/shipping.

It is hard to get hold of new ones in some parts of the world; personally
I'd love a new F60 but can't find a supplier in the UK at a reasonable
price.

-- 
pondlife