Is MacBook Air a reliable machine ?

Hello.

I'm an Electrical Engineering student and I badly need a new computer. My old Lenovo is 5 years old now and still going great ! It has a pentium dual core era processor with just 2.5 GB of DDR 2 ram. The thing is that it's big ,heavy and ugly and it can't run MATLAB and other cpu intensive things really well. Not to mention the 30 minute battery life =)..

I'm in the market and i'm really looking at the Macbook Air. I have a few concerns however :

I live in Pakistan and I can't afford a computer every 2 years. I need this thing to last me 5 years or more. I need it to be reliable. I take reasonably good care of my machines i.e I don't throw them around or use them on mattress (causing heat up) . Keep in mind that the summer is quite hot here with tempratures going up to 45C. I don't always have air conditioning available.Furthermore ,I like leaving my laptop on forever. I have torrents and stuff downloading constantly and my lenovo has held up like a tank for that sort of usage. Is a mac too fragile or 'weak' for this kind of stuff ?

2- I have an iPhone 5 and the thing is as fluid as butter.When you click something on the phone it just responds ,it's fluid and makes you happy every time. There's this certain confidence with which the phone responds to you and you never really fear any lag or stutter even when you have lots of ram heavy apps in the background. Do macs work the same way ? If I'm spending a premium on a machine I should expect that level of performance right ?

No fanboyism please.

Thanks !