I'm probably pushing the 25% range. In a 'Mech only engagement in the 3025 era, they're potentially useful for overheating enemy 'Mechs and starting fires in the opposing cover. I'll usually take them if I have the right unit in the group that can make good use of them. In a 3025 combined arms scenario, they're especially devastating, and a much higher priority. In a 3050+ 'Mech-heavy scenario, the double heat sinks tend to make them a bit underwhelming, and I rarely use them in that case.
My favorite platforms are the venerable "Thud" with the SRM-2 rack, or a Scimitar with the AC/5 and SRMs. Normally, I'll pair the Scimitar with a couple of Saracens sporting LRMs and regular SRMs, so the Scimitar gives me "aimed shot" direct fire capability and a bit of warm and friendly Inferno love to share.