I noticed this same combo WAAAAAAAY back when S&F was released. As a primarily DM I had some concerns, but the S&F errata solved the cheesy-loop issue.
Recently I played in a campaign and one of my PCs, a heavily multiclassed 'Brawler of Kord', used this tactic to good advantage. The key thing to keep in mind, as many people have pointed out, is that Tripping is rarely effective vs burly or dextrous opponents regardless of thier level. Its also a resisted roll, so even vs opponents that you have an advantage against you can roll low and biff it.
My character eventually got a Belt of Strength +4 around level 12 and after that he was able to get some use out of the feat. As someone has already pointed out the primary advantage is to deter full attack actions by opponents, but it is primarily a defensive action. Because capability doesnt degrade with injury in d20, it is generally always better to terminate one opponent than wound several opponents.
The DM wasnt happy about the feat because I would put opponents down like bowling pins, but he never seemed to catch on that I ultimately took a lot of damage in the process because I was taking on many opponents simultaneously to get use out of the feat chain.
The most important thing to remember is that WWA is actually a means for the DM to manipulate characters that have it. Unlike almost all other feats, WWA forces the user into a dangerous situation to use it: get surrounded by enemies; the more the better. This is where the feat is counterbalanced moreso than the feat chain necessary to get it. As the risk scales (more opponents) the gain potential does too, but the feat doesnt scale vertically (relative power of opponents) very well. Thus, it is most useful vs opponents that are already not challenging. With a bit of play it becomes obvious that WWA is simply a Chump Drop, just like PA->C->GC and Ice Storm. Ideal for use vs weak opponents that just get in the way, not so great vs tougher opponents. Due to the nature of the CR/EL system such opponents as are most affected by it will yieild the least XP.
As far as your Devil issue is concerned, IIRC, Flying creatures cant be tripped (??Somebody pls Confirm/Deny). If this is true, your Devilish antagonists could fly 1 foot off the ground; fighting for all intents an purposes as if they were standing, but untrippable.
Finally, as a DM and a rules-tweaker, my primary complaint with the Knockdown feat is simply that it takes an oppossed die roll _and_ because it is an oppossed roll the probability matrix on it is extremely spread. Im considering altering Knockdown to either be resisted by a Str/Dex check vs a flat DC = Damage Inflicted. I am still pondering it.
