Welcome to Laptop GPS World.
I never heard of SEJAT SJ-5210 before
It is about $10 less expensive than the very popular GlobalSat BU-353.
...and uses the latest SiRF Star III chipset.
SirfStarIII has been around for over 5 years. It is definitely not the latest chip used in GPS receivers. Nevertheless, SiRFstarIII is still a very good performer.
This unit is much more sensitive than alot of other receivers.
It may appear like that to a user, but without actually doing a side-by-side comparison(s), user's first impressions may not necessarily tell the whole story.
Before purchasing this item, I read all the reviews I could find
I've searched, but the only reviews of this unit I can find are reviews posted by you at various sites. In fact, based on what I've read, it appears that you yourself sell this new and unknown USB GPS receiver
If you want a good WAAS enabled GPS receiver for a laptop, this is it.
While WAAS is often an item emphasized in sales advertisements, this feature is of 0 (zero) benefit to people using a GPS receiver for in-car navigation.