There are a couple major cons of the battery packs that would make me not want to use them, other than the fact they are generally more expensive than NIMH AAs.
First of all, I've never had a pack outlast quality AAs, rechargeable or not. If you do a google search, you can find some articles about people opening these things up. Whats inside? 2 no name brand NIMH cells, which are only 1200MAH. (I've heard some newer packs have 1800mah cells but I'm not sure, guess it depends who is providing them to MS) On top of that, they are non LSD cells, meaning that if you let the battery pack sit for about a month, it won't have any power left until you charge it.
Secondly, they batteries degrade very quickly. These are not quality cells you are paying for. MS gives the contract to the lowest bidder. I once owned an MS battery pack, and when it was brand new I did get 15-20 hours out of it. That lasted for maybe a year, at which point I was lucky to get half that. By 2 years I was getting around 2-3 hours out of it and then it died a few months later. Now it' a paper weight.
Thirdly, and I can only vouch for the official MS pack although I assume others would be similar, is that when the pack is plugged into the controller, it is always on. There is always a VERY faint light on the controller with the pack inserted, and it will continue to use a little bit of power all the time. If you leave it plugged in to your controller, it wont last more than a few days. This does not happen when you put AA batteries in the controller.
Now, does all this apply to every battery pack? I don't know. But why waste time and money finding out when you get get 4 Eneloops plus a charger for around $25? Keep one set charged at all times while the other is being used. And the best part is when the Xbox 720 or Wii 2 come out, these same batteries will likely work just fine for those too
Oh, and if you still would prefer the battery pack for whatever reason, don't get the plug and charge kit. It will kill the battery pack very quickly. The official Xbox 360 charger may be expensive but it should properly charge the pack.