It is hard to say - you can get a touch-screen, you can go without any ...
Depends on where you sail.
I use a Garmin 72 and it is great - good enough for round-the-world.
Would I buy one of the 620/640 series? Well, depends on the cruising ground. I love them, I adore them, I desire them, but I do not NEED them ;-)
If you are in a fjordland or an area with a lot of shallows, etc. it makes sense to have a plotter (a GPS
unit with an e-chart in it - it shows you your location against the e-map and plays other nice tricks (like showing AIS
equipped ships) too).
If you are in an area where navigation
is easy and you do not have to many hazards/fog ... why bother?
Just make sure you have a compass
, good binnocs and other 'regular' stuff before you go for a GPS