I've used a few Portable ones and found them to be suprisingly very good.

How is it they are as good/better than the D3 for GPS naviagation when they don't have to be connected to VSS and there are no issues about antenna placement, angle of set-up etc.,

Just curious. I'm happy with my D3. It just seems odd that these little portable units perform so well without the issues of set-up.

Because they are specialized units, and much more research has been put into the effectiveness/accuracy/ease of use of the GPS because of that. With the D3, that research and development time was split up between all it's various features.