Do NOT use a different power adapter other than Apple
I wanted a 2nd power adapter in my backpack cause my powerbook is fairly on my desk same as on the road. I would feel better having a power adapter on my desk and one in my bag. The Apple Power Adapter is awesome, but fairly expensive ($80), so I thought I could look for some alternatives.
The MacAlly one WILL NOT be sufficient for your Powerbook 17". The current MacAlly one says it can power iBooks and Powerbook G4's, but they are speaking of the old Titanium powerbooks. The MacAlly adapter only spits out around 45-48 watts of power, where as in the new powerbooks you need the full 65W the Apple adapter provides. I plugged in the MacAlly power adapter and the icon changes to indicate it is plugged in, but if you start using power hungry applications like DVD player or gaming, you will notice the percentage drop as if it were not plugged in. The only way for it to even charge is for the entire notebook to be off.
A worthy, but expensive, alternative is the iGo that is sold Radio Shack because the unit will produce about 70-75W. The only advantage of this is that it is expandable to not only charge your powerbook, but also your cell phones, other laptops, and gadgets. If another company offers a power adapter for your Powerbook, please make sure its around the 65W zone because your Powerbook needs all that juice.
My advice to you is, if its an option for you, check your local Apple Stores this weekend for it is the last weekend, and they have the "Fire Sale." I was really lucky to find power adapters for my 17" Powerbook (2 of them actually), refreshed for $29.99 each.. marked down from $79. Now I have one on my desk, one in my bag, and one in my drawer if I want to move around the house.
Apple refreshed/refurbished products are just genuinely awesome.
Hope this information helps you out and happy findings.