Is there any reason to invest on GPU based system these days? (March 2013) I wonder whether I should spend around 1000$ - 1500$ for buying a GPU based system that will mine at 2GH/s. Is it still worth it? What will happen when all those ASIC machines will arrive? will i still be mining at a decent rate?

Thanks.
As of today (April 6, 2013) mining profitability on GPUs is at the highest levels ever, but that is only because the exchange rate has rocketed up so fast.  


http://blockchain.info/charts/miners-operating-profit-margin


At the same time, mining difficulty has been skyrocketing and that pace of growth will likely outpace the increase in the exchange rate.


http://bitcoin.sipa.be/speed-lin-10k.png


But there are probably two or three difficulty adjustment cycles remaining where mining bitcoins using GPUs is still profitable.   Enjoy it while it lasts because in a little over a month it should be all over for GPUs.   The expected rate of difficulty increase was not unexpected so further investment in GPU mining has been discouraged however the rise in the BTC/USD from the low teens to 10X that level has caused that call to be simply premature by a few months.