All the advice is great - I have a Stihl MS171 - great piece of kit. I also have a couple of bigger chainsaws, one a Qualcast which has been so good I brought another 2nd hand for spares for when/if it goes wrong. I cut about 4-5 tonnes of logs each year, and really enjoy using it.
I have found most chainsaws are good enough if you look after them - definitely use 'Fuel Fit' which make the new crappy petrol much better and do not leave them for a long period with fuel in them (tank or carb). I also find the fuel still rots the pipes after 2 years so always have spare fuel pipe & filters in the toolkit. Some of the cheaper ones require a 're-tune' depending on the temperature - My Stihl has never needed it.
I also have the 2in1 sharpener, not the Stihl but exactly the same, even the colour, and an electric one - the 2 in 1 is much better, quicker and does a better job. Best indicator of a blunt chain is the sawdust little or very fine is an indicator of a blunt chain (and it doesnt cut well!!)
I would also agree that the safety kit is essential, especially if you only use the chainsaw infrequently, and definitely if you use it a lot.