Somewhere between good and great IMO, obviously not for every ship but S/M energy doesn't have any alternatives for what it does.
On Executor it's straight silly, against 1200+ Ordos with no playership they routinely get >250 pro rata DP, despite Executor often turning off weapons while still at ~0 flux.
When HEF is up a brilliant's entire hull is gone in seconds.
Generally looks something like this:
(friendly cruisers leaving it with pants down in the front against 2 radiants is also typical, need to tweak my Eagle loadouts
)
There is a big difference between this and thumper, namely this has 5 times the hit strength vs armor.
1000 range and perfect accuracy helps a bit too, and it's a pretty good fighter-swatter in practice.
1000 range, perfect accuracy, instant hit, not wasted on shields, very fast turnrate, M energy instead of M ballistic, 2.5x hit strength (beam hit strength is usually half of DPS, somebody checked and for autolance it's also 250).
So yeah, nothing like thumper