Still worth to buy a Mining Rig?

would say no. I mined Bitcoin and alternate coins for quite a while a few years back, and even then it was really hard to make a fair amount of money with it.
However, if you can find the right coin and play your cards correctly you may be able to turn a profit. You essentially need to find a relatively new(er) coin or one that has a lower difficulty level that you believe to raise in value. Once the difficulty level has increased quite a bit, hold onto the coin and move onto another.
After you've developed a pretty good collection of several different coins, (and they have gotten to a suitable value) sell them. This is what I did a few years back and it worked out quite well.