I have been using my MB everyday now for about 3 weeks.
Coming from a quad core MBP 2.6i7 16gb Ram Nvidia graphics card etc I was similarly sceptical about performance on a daily basis, but was drawn to the lightweight portable design as more and more of my work is done on the move.
I use it everyday professionally for Unity app development, Xcode software development, Dreamweaver web development, Photoshop image editing, Modo 3d modelling, ZBrush 3d modelling, Sculptris 3d modeling and tons of other things besides and the MB excels in every task I give it.
Worrying about performance really is a non issue - which in itself is amazing.
Obviously as time goes on, you mention 5 years, hardware will get better and things move on to the next great thing.
So committing to something now expecting it to still be great in half a decade is foolhardy.
I felt your major concern was the price. If you are considering buying the MB on 5 year finance then don't! Buy something more in your budget, work hard and then buy the MB down the line if necessary. It is always horrible to hear stories of people over stretch themselves for desirable kit they really can't afford and then have to sell to dodgy people for less than they paid for it to cover debts.
If I have made a misjudgement and money is no issue then go for it. You won't be disappointed by performance versus the portability this little machine gives you.