It depends allot on the motherboard you have as well. For instance, My P35 DS3R will NOT run external 2.5 inch drives on its front ports(the back ones are fine, guess they want to play it safe, they have full control over the traces on the board that lead to the rear ports, but not the wires in your case). The current limiter kicks in @ 2.5 watts, yes its only 2.5 watts not 5watts. 0.5A X 5V = 2.5watts
That said my old compaq R3000, H55n-USB3 and every Asus board i ever owned power just about anything off its usb ports. Even ran a satellite radio unit rated @ 2 amps on it
Either way, with those Y cables you should have no issues with any 2.5 inch drives.
Every 2.5 inch drive i had only needs this power to get spinning, after that, one usb port was able to keep them running along.
For my money, I would go with ESATA/External power(3.5) for speed every time(desktops). For a laptop, USB/Power-ESATA(this is a usb/esata port combo) or USB3(it has more power and speed, no faster then esata with current drives, but may become standard soon)