I love it.
For me, it's not so much that its 'fresh and stylish' or 'old and clunky', it is just a powerful feminine name to me. It also seems 'Australian' to me due to 'Waltzing
Matilda' and I like that it has multiple nickname options.
I'd prefer
Mattie or
Millie as nicknames but I'm not opposed to Tilly...it just isn't my favorite. For me and the way I shorten things, the most natural nicknames to come from it if I was saying the name all the time would likely be Mils or Tils anyway.