I've been using Garmin for years and would thoroughly recommend them.
On the few occasions I've contacted customer service with queries, they've always been very helpful.
I have a nüvi 660; one of the reasons I chose it over tom-tom was the higher quality of look and feel which it had. I don't normally bother with voice commands as the magenta line and screen prompts are good enough IF you have a dash-mounted system:
I don't like windscreen sucker mounts, so installed the dashtop 'automotive' mount. But if I'm in Europe or North America on business, I take a windscreen mount and fag plug power lead with me so that I can use my familiar nüvi 660 in the hire car.
Were I to buy a new GPS, it'd probably be a Garmin aera which has both automotive and aviation modes - and fits my existing dashtop mount.
Although my nüvi 660 doesn't have an 'INS' included, it does have 'road following' which, in 'Faster Time' or 'Shorter Distance' navigation modes stabilises the present position to the road map if the satellite signal is problematic. If you go into a tunnel, the system maintains the last known velocity, so it doesn't get totally confused. I was in a tunnel in Germany last year, with a road junction in the tunnel and a significant number of bends - but the nüvi caught up as soon as I came out into daylight again.
GPS has really taken the stress out of motoring in unfamiliar areas - I don't know how I managed before I had one. I bought a nüvi 360 at Birmingham airport some years ago and read the manual during my flight to Frankfurt and thence to Bremen. Got to the hotel and connected it to my laptop to register the system, download a software update and initialise the TMC subscription. Just as well, because the next day we had to drive from Bremen to Dresden on unfamiliar roads. Funnily enough, although I flew back to the UK from Dresden, my colleague without a GPS took a wrong turning in old East Germany on the way back to Bremen and wasted over 3 hours....