I have a 600 (same as 650 w/o camera)...a few thoughts:
The screen is 4", but the resolution and brightness don't come anywhere close to a smartphone or even a newer GPS nav unit. Lower your expectations

The navigation is poor compared to standard navigation software on a smartphone like Google Navigation. It's 'ok' compared to a Nuvi unit my friend has. I really don't like the screen, but I'm spoiled by smartphones

Battery life on my rechargeable battery appears to have lost quite a bit of power in the year+ I've owned it....last full charge with the screen on fulltime, full brightness, I got <6 hrs.
Topo maps are solid for a GPS unit if you get the detailed Garmin 24k topo maps and load them on. If you load any other overlays (ie a trails overlay), the unit tends to slow down significantly as it has a hard time rendering detailed topo maps w/overlays (it's better if it's zoomed way in and doesn't have much on the screen).
It's waterproof and really durable
I picked mine up for $300. At that price point, it is BARELY worth the money, especially given the exponential improvement in smartphone quality. It's almost better to go with a used unlocked smartphone and load on BackCountry Navigator and not even sign up for a cell plan

Throw it in a ziplock bag to save from the elements and you'd almost be better off.
The most recent trip I did was over Labor Day weekend, used a smartphone + the montana...honestly, I used the Garmin more, just because it was more durable and I was less worried about breaking it, otherwise, I'd rather use my phone...