Mechanically speaking, they are okay if you use their errata'd version (merc longsword 1d8/x4, merc greatsword 1d12/x4). Be prepared for some exceptionally mean crits once in a while, but 95% of the time it won't really matter.
They're pretty silly as anything but minor/major artifacts, though! I can see such weapons adding a bit of excitement to a high-level game where the fighters might be wondering what to do with their last 4 bonus feats. I don't really see level 1 characters running around with them... sorta like children playing with actual greatswords! Even the Long Sun series (which was pretty much an "anything goes" campaign) mostly limited such weapons to beings that already had a high degree of combat ability.
In my campaign, the famed "githyanki silver swords" are swords of this type. If you own one, a bunch of githyanki are after you!