Yes, Towelroot is safe. There are the same security concerns that you would (or should) have with any rooted phone, in that you can easily give dangerous permissions to sketchy apps, but Towelroot itself is just a means to an end. I currently have an LG Volt Android phone that I rooted using towelroot. I've rooted every Android phone and tablet that I have ever owned, and using towelroot was the easiest method I’ve ever found for rooting. By far.
From what I understand, recent updates to Android can close off the method Towelroot used, so it may not work for much longer. The developer behind Towelroot, Geohot, is well known and respected in the developers’ community. This is the same guy that unlocked the iPhone and figured out the PS3, so he/she has a good track record.
I’ve used Towelroot without any problems, and I would use it again without hesitation.