I used them in the past, and would still use them for my son. I bought a tux a few years back. They aren't that expensive, and you can recoup your costs fairly quickly.
A Cruiseline Formalwear story - Before I owned a Tux, I rented one from Cruiseline. It was delivered to our room, and I gave it the quick once-over to make sure everything was there. So, formal night rolls around, and I start putting on the Tux. I pull out the shirt and was shocked to discover that the shirt is custom modified for a man who I can only assume was an amputee. One sleeve was significantly shorter than other (above the elbow). This is when I discover/realize that there is an entire tux rental "shop" on board. Within 15 minutes I have a new shirt in my hands. My only hope is that the person that this shirt was for was able to get it in time, assuming they were on our cruise.
So, is there no Tux rental available on the ships anymore? Has anyone taken over?