2nd to that!
I'd say the best way to simply sum it all up is that stainless is a little harder to work with, use can not use a Oxy/acy, torch and cutting with cut off wheels and / or drilling with be more laborious, but as far as welding to mild steel you will not have a problem.
If you are not using 100% stainless steel for the entire construction, the advantages of using this piece are depreciated, and the disadvantage's of the extra labor required are less justified.
But its free..and that is a big advantage, so snag it up and if you can not find anything else that fits what it is your trying to build, don't let the fact that its stainless intimidate you into not using it, but at the same time, don't let the fact that it is stainless allow you to glorify the part and to work around something that may alter what you intended to build in the first place.
Regardless, building something like this is labor intensive, its better to take your time and get the right parts for what you want to build instead of working around something that's free. There is nothing more expensive than wasted time!