So I've been riding my $125 eBay purchase Sundeal F2, 8-speed, folding bike with disc brakes for the last 7 months.
My commute on it 4 times a week is approx. 6 miles one-way, which include going through the NYC subway turnstiles.
The bike is working out great.
I have wore down the foam grips that I changed to 6 months ago, mostly because I use rubber bands to hold down the front brake when bike is inside the subway train.
I also changed the seatpost to a carbon one, to reduce the weight of the OEM aluminum seatpost, at 650mm length, that's quite a difference.
Narrowed handlebars to help me navigate among NYC traffic.
It's been a reliable and decent ride.
The disc brakes are mechanical (non-hydraulic), but enough stopping power and once setup proper, they never seem to rub.
Just recently they started to make noise when I come close to full stops, may just need to clean the rotors & calipers.
All in all, very good value for the purchase price.
The mini-velos have not been used as much as the Sundeal folding bike.
Mainly because my commute also involves subway rides.
If I plan to rider longer distance than my regular commute of 6 miles, I would ride the mini-velo instead.
The longest rides I've taken on the mini-velo is about 40 miles, I think I plan to put a riser handlebar to replace the flat bars on them currently.