I've been playing Diezel amps for several years, Herbert is my main amp of choice, and I never looked back to anything to replace it. I Had Diezel VH4, Mesa Mark IV, Mark V, Rectifier, also tried Mesa JP2C, Soldano SLO100, Friedman BE100, Bogner Ecstasy, and couple old JCM800 – Herbert is still the king, at least for me.
The main reason why I bought this plugin, was to have some little travel/practice rig, so I can play through similar setup when I don't have amp available, and the other reason is not to burn tubes for nothing, when I just need to practice some licks.
After hearing some demos I was very sceptical, for me personally, none of amp modellers will ever completely replace tube head. But after testing it extensively for couple weeks, I have to admit UAD Herbert sounds pretty good, behaves more or less same way, as real Diezel Herbert. off course there are some digital artefacts, and less 3D comparing to real amp, but for what it does I think this UAD simulation is pretty good.
Other reason why I find this plugin appealing is because it looks like real amp, approved by Diezel himself, so the is no silly made-up name ala DZL, Benzin or violation of intellectual property etc.
I wasn't crazy about build in IR's of Diezel cab, I think they OK, but to my taste OwnHammer IR's of the same Diezel cab sounds better. It would be great if UAD will make some kind of external IR loader, so everybody can use their own IR's with low latency Apollo interface.
Also have to say that, there are many other plugins that sounds as good as UAD Herbert(for example Poulin Lepou makes some great and free stuff, Nembrini SLO100 and others.
So here is my quick comparison, Amp and Plugin recorded at the same time, guitar(Hamer Californian) goes to Fractal FX8(which is only for splitting signal) then one part of signal goes directly to UAD and Herbert Plugin, the other one goes to amp, then Suhr reactive load and UAD card. I didn't use Stock IR, because I think that IR of Diezel cab loade with Eminence Governor (OwnHammer) sounds way better. Some delay, and modulation done by Eventide H3000
https://youtu.be/g97mYJnYNB8