I have had a chance to do some field-testing of my new BU-353S4 now and I thought I would share some of my observations. First, I should make clear that I cannot compare it to an S3 version because I do not have one. So, my personal benchmark would be the GPS receiver that is built into my Viliv S5. Second, I should point out that this is in no way a balanced test; all terraines, all conditions, etc. However, I did use it in a fairly hostile environment: Manhattan. I say "hostile" because the tall buildings we have here are not friends of GPS. When they are not busy blocking out the GPS signal, they are distorting it as the signals bounce off their large, flat, vertical surfaces.
The S4 definitely out-performs the Viliv, however. An easy indicator is how many satellites are being recognized. For instance, parked between two 50-story towers, the Viliv showed 4 where as the 353S4 showed 7, 8, or 9.
The S4 produces a relatively true and straight GPS trail that seems to be pretty accurately positioned. You can even see where I had changed lanes. No, I did NOT cut off that taxi! I was there first. Even on relatively wide streets with low-ish buildings, I would get a sort of saw-tooth effect on the GPS trail with Viliv as the recorded trail was distorted by varying signal qualities.
The start-up seems almost instantaneous. After being on standby and resuming the GPS navigating, I had a red dot before I could get the key in the ignition. This is a major boon.
Also, the location lag seems to be substantially reduced. In other words, the red-dot or arrow in Streets and Trips is much closer to where you actually are rather than where you were 5 or 10 seconds ago. This can be a real help in giving driving directions in unfamiliar territory although I have not actually tested that.
So, in summary I am favorably impressed. I cannot think of any negatives. Get one!