Hello and happy New Year
I own a first gen 2006 Er6-n bought brand new ten years ago. I can walk to my job so I don't use it every day as a commuter yet it has never stayed idle for more than a few days as it's ok to ride during the winter here. So I mostly do sporty riding, at least a couple of track days per year, one week long European trip per year and a lot of 90 mile commuting to my summer house almost all year round.
I've done pretty much everything with it except motocross, a few months ago I crossed 9 countries in a week with it with no problems at all, I've crossed the Transylvanian Alps, once I did 1146km in one day form Croatia to Albania, back to back weekend trackdays and the next day this amazing bike was 100% ready for more
I love it and totally recommend it!
First gen models are slightly lighter and more powerful, better set up and nimbler but they are also more vibey and the they feel coarser in general when compared to newer models.
Common problems include fuel pump trouble and stator coil - alternator failure. I've ridden mine through appalling conditions without even a hint of electrical problem, in fact I've only changed two bulbs in ten years. There is also a slight oil leak problem from the water pump assembly but most cases were resolved by replacing the pump, the shaft and the gaskets. All these problems mainly, if ever, occur usually after 36.000kms. I've seen bikes with 90k+kms with no major issues though.
So if you buy a used one I would suggest a full service, a fuel pump fuel rate test and probably a stator coil replacement before setting off since these are the main problems that can render you immobile and are hard to fix on the road.
Also, the first gen models have an extremely soft front end and the brakes are more than adequate but without much feel on the lever and you could change the suspension and braking software once you get used to the bike.
PS: Click on my username and go to statics, there you will find a few threads and posts I made through the years with interesting stuff about first gen models