I have a build-in system in my car, so that will not help you finding Knoebels
What I can tell you is that during our recent trip in the Usa, we had rented a Garmin gps and it brought us just fine to Knoebels.
It wasn't really slow either.
I can't recall the exact type of it though but it looked like it was already some what old. So I guess the newer Garmins should even be better.
So far only got stuck on a ride and that happened only recently.
During morning ERT at CP we went at 930 h on Maverick. Just before re-entering the station, the ride broke and we had to wait for 30 minutes before we would get out. So we've spent 50 % of the ERT stuck on Maverick.
^ Yes, buy a flash pass (usd 10) and you will have time left.
Not sure whether they sell it online but if they do, I would buy it online as when we arrived at 230 pm on a Saturday, all flash passed were sold out already.