I have never tested them so I don't really know about their quality, but anything that claims to be the "Cheapest in the world!" should be treated with a bit of skepticism.
On quick but important note, you should avoid using analog cards like these(and Digium/Sangoma/Rhino analog cards too) fo use with VICIDIAL, you will have problems and you would be better off using SIP trunks or T1/E1 circuits.
now on to warranties, this is a lot of what you pay for, service and warranty. Sangoma recently raised their warranties to lifetime, meaning that if the card ever goes bad EVER they will replace it. That costs a lot of money to do if you make an inferior product, not much if you know that your cards are top of the line. A 2 year warranty is about a low as you can go. Digium and Rhino both offer 5 year warranties on all of their cards. Of course for the cost of a single Digium/Rhino/Sangoma card you could buy 4-10 of these NXTVOX cards so you wouldn't need the warranty I guess
