I'm using Debian wheezy and i wonder are Ubuntu PPAs hosted at Launchpad safe to be added & used on Debian?
Regarding safety I'd say they are even more unsafe and more likely to break your system than they are on Ubuntu. Consider a badly written package, that does not list the correct version of a required library as a dependency and therefore installs, but doesn't run. Now imagine what happens if this package is a core system component. So, if you're planning to use a repository of a different distribution, think twice and double check before attempting to install anything. Read carefully through the list of packages that are going to be installed to make sure that no important system components are being replaced. If you are planning to replace system components I recommend to set up a virtual machine with Debian and try it there first.
Nevertheless, the real problem is, that packages containing precompiled binaries will most likely refuse to install due to dependency problems anyhow.
I don't know about Ubuntu 11.xx and earlier, but Ubuntu 12.04 has libc6-2.15, while Debian Wheezy uses libc6-2.13. Packages in Ubuntu PPAs usually require at least the libc6 version of the Ubuntu release they are built for (because they are also built with the build chain of that Ubuntu version). So, although you could simply click on the "Technical Details" link on a PPA website to get the deb repository URL and add it to Debians sources.list, the packages in up to date PPAs will likely be useless on Debian.
If you are planning to install packages that do not contain any precompiled binaries, the situation is of course different and they will likely work.