No, Ragnar is correct. The problem with the GPS in the N810 has nothing whatsoever to do with AGPS or no AGPS. The problem is simply that as a GPS receiver the TI chipset in the N810 is not as good as e.g. Sirf star III or the MTK (sp?) chipsets.

Assisted GPS is just a way of feeding pre-loaded and pre-received data to the GPS, to save time (time that would otherwise be spent waiting for initial data from satellites) and, less important, some power (for using the data to calculate some stuff). AGPS can be useful with any GPS, including the fast, mega-multi-channel ones.

As for 15-20 seconds lock time: You can't guarantee a lock time that fast if you start from cold conditions. With AGPS and a chipset like Sirf III you could get 1 sec lock time at all times, for example.

Again, the problem with the N810 GPS is that the chipset isn't that good. AGPS makes it less bad. However, the TI chipset probably uses less battery than a Sirf Star III would (not that the latter one is bad in that respect though).