Yes they do. With GPS only I never had a situation where I could "see" more than 6-8 sats at once with a mobile and not all of them had a fix/lock because the signal was too weak (only once supposedly had 9, reported by an Audi in-car satnav with dedicated external antenna, but I am not sure if that was fixed sats or just the visible ones). I think as of now I think there are less than 30 functional sats in orbit globally and the GPS architecture hovers around an amount of 32-36 sats maximum (with higher coverage of landmasses and lower coverage on the oceans).
Also, GLONASS will be very beneficial to anyone in the upper northern hemispere, and lower southern hemispheres as the system is of course globally active, but due to sat orbit inclination and distance (orbit heigth) it allows a better coverage of the regions closer to the polar regions, while GPS seem to be more designed to cover the "middle ground" and also have a lower orbit.
And when the US went to war in the middle east, I think I have read about some of the satellites being repositioned to give a better signal feed to the troops and equipment in that area, therefore automatically neglecting the rest of the world a little bit.
This is a really good feature I did not even read about in the reviews prior to purchasing. I pity the folks with the One X who don't have that... It will double satnav usability and accuracy even in less than optimal conditions (cities with high buildings covering satellite line of sight etc.)