I've been a member of the website GetASlogan.com for most of a month now, and in that time I have not been impressed with the site at all. Since I joined, only one slogan contest has been posted that qualifying writers could submit to, and even though the site says that all qualifying slogans will be posted on the website for the public to see, there are no slogans shown for this contest, which has now been open almost three weeks. Even the three slogans I submitted aren't shown, and there's nowhere on the website where I can see my entries. The contest is apparently closed, because I can't enter any more slogans, but no results have been posted or announced.
Even when the contest was open, it wasn't listed under my Active Projects, where you would think it would be. The site is not very intuitive to navigate. The site seems more geared toward clients than toward slogan writers, which is great if you're a client. Slogan writers have to qualify in order to be able to enter all contests, and until you qualify you can only enter certain contests and can only win smaller prizes.
GetASlogan.com is free to join, so it can't hurt to sign up and see what it's all about, but from what I've seen so far there doesn't seem to be much to be gained from it as a slogan writer. If you're looking to make money writing slogans, you'll be better off checking out SloganSlingers.com. The pay is generally much higher, you'll only be giving up rights to slogans that are actually purchased, and the whole process is much more transparent.
For more information about GetASlogan.com, including specifics about prizes and payments, you can read their online FAQ.