Yup ^^ Ol is 1 of the most versatile class in the game...you have all kind of debuffs to manipulate your oponent, you have life drain (I dunno the right name) and have clan/alliance buffs...the point is that WC have party buffs, what is easy to lvl in party, and OL is hard...
Another point its the drain life magic only drains back hp to you when you drain enemy's HP, not CP...if the other person beggin using frenetic cp pots, you'r in truble (sorry, sometimes I can't explain clearly in english)
OL is excellent for mage groups. I've gone with OL many times (when on my SE), instead of a prophet, and it works well. Both for acumen + zerk (and defensive buffs and ww) and HoP for the body to mind'ers. Of course prophets have their nice special buffs, but OL has the necessities.
OL is an alright secondary buffer for melees/archers, but obviously lacks the essentials. However, it takes some strain off the other buffer in party.
OL and WC share the same kind of attacks (hammer crush, steal essence). Their diference are in the WC DOT's and OL's debuffs, with the later being much stronger (if WC is silenced, then bye-bye). The WC's freezing flame and aura sink can't really compensate them. Another advantage of OL's is his CP (at level 70 almost 1000 CP more than the WC). So I'd say that at equal level a OL wins most of the times a WC.
On the other hand, everybody says that leveling an OL can be really hard, while I can't say the same for my WC (I frequently need to turn down party offers, beacause I already have one)
Overlords become Dominators which give acumen, zerk and empower3. On top of that they are great for crowd control. Having an OL in party for AOE parties in Forge of the Gods works wonderfully. AOE root once the mobs bunch up has saved many a mage. :)