I got lucky once with an accoustic. I was attempting to line up a task force north of Norway in mid '41 and was about 3500m out when one of the escorts detected me and came after me. I did a quick 180 degree turn then fired off an acoustic at the DD, but he slowed to try and pick me up again and the ell just cruied staight past him towards the Rodney class BB that was making lots of noise at 21 knots.
BANG!!! The acoustic eel knocked out the Rodney's props and she slowed to around 1 knot. I hung around until the rest of the escorts gave up looking for me then snuck back to PD to finish her off.
Otherwise I've found them great for knocking off annoying DDs from the stern tube.
The ladder eels I keep exclusively for convoy action from the bow tubes. They work well when you have to make a snap shot and don't have time to get all the calculations finalised but only if there is a lot of traffic about.