I guess it depends on how much the £/YTL exchange rate matters to you. If you are here full time, it is fairly irrelevant on a day to day basis (how many pounds your lira's are worth) because you are saving & spending lira. For people looking to invest in YTL while living in the UK, it can be something of a rollercoaster as the rate against the pound goes up and down from 2.1 to 2.8 (approx, over 3 at one time recently). If you need your sterling investment back at a time when you need a lot of lira's to buy a pound, you could have wiped out anything up to 25% of your initial investment. Of course, you could also make up to 25% if you invested in lira when the rate was high(to the £) and you can cash in at a lower rate.
As I see it, the YTL pays good interest because there are risks. Short term exchange rates can change dramatically in no more than a month or two. That said, long term the rate has been pretty stable for at least five years. It's all about how many lira you got for your pound when you bought in, and how many lira's they give you for your pound when you want out.