The Zooms are incredibly popular, I doubt the numbers being sold second hand on ebay represent the greater number of them being sold. Some people probably want to upgrade, find they didn't use it or need the cash.
There are plenty of comparison videos on YouTube comparing the different zooms and their competition, quite often with singing/acoustic guitar, which can give you an idea of how well it will record your uke. Like anything, it may come down to personal taste.
I've been doing some research into portable recorders, and I think the sound quality on most of them is pretty decent (where some seem to fall down is recording full amplified bands, but that's the pure volume probably), it's just making sure it has the features you want/need.