The Lookdown - Is it any good?

oh no, an ability for which you have to use your brain ! This is so terrible...

If you're looking for a strictly Yes/No answer, then No.
The Lockdown isn't that good.
(OP seems to only be speaking of the MAX version. The Prowler's Lockdown is a little more useful but not that great still)

Both ZOE and Aegis can be turned on/off repeatedly and instantly (well, near-instant with Aegis)
Aegis shield needs to have charge (i.e. not pulverized) but can be used while reloading.
MAX Lockdown is slow to deploy and undeploy. Hell, you have to stop moving first to even activate it,
further slowing the process down.
It has limited situations where it can be nice but it usually gets me killed more than it helps.

With lockdown you use your brain and equip charge or even better play VS instead. huehuehuehue

Stealth (a must), AP and Halberd and a general idea of the layout of the location you are in so you can flank = Enemy tank, sundy lightning heck everything killer (just keep eye out for ESFs, a lockdown prowler is an ESF lolpodders wet dream).

Every other situation use at your own peril