Hi Nina,
I don't like Macally enclosures for their lack of a simple on/off switch. A momentary tap to power switch usually means it won't recover from a power failure on its own. Moreover, they are also more sensitive to power surges. A surge that destroys a sensitive momentary power switch will continue to destroy the sensitive electronics a real current - interrupting power switch would have a better chance of protecting.
I have had many problems with consumer electronics trending toward the cheap route, saving $2.50 by their decision to eliminate real power switches from all sorts of devices from appliances to home entertainment equipment that costs real money. Many problems. As much as I like Macally, this is a deal breaker. No question.
I have several Acomdata 2.5 inch drive enclosures. They're simple, inexpensive, and suit my needs. I do not have any of their 3.5 inch enclosures, but if I did I would get this one:
http://www.amazon.com/Acomdata-Firewire-3-5-Inch-Enclosure-SMBXXXU2FE-BLK/dp/B00 1PN3QCU/
Wall wart: yes. It will be for a G5, therefore who cares. For portables, I like bus powered, but for desktops, I prefer a power supply. A real power switch is a plus. Low cost: definitely.
FireWire means there is no question it can boot your G5. No FW800? FW400 is fast enough for me; it may not be for you. I routinely boot desktops with USB 2.0 and I don't understand all the fuss about speed. USB 2.0 is fast enough and FW400 is even faster.
So much for my 2¢ contribution. Keep the change.