I use the Specular Tempus for my synths (Sub37, NTS1, MicroFreak) in the studio. Both the reverbs and the delays sound lush and absolutely gorgeous. Coming from a (now broken) Lexicon multi-FX unit, this one sounds a lot more modern but with the same "quality vibe".
I initially bought a second hand Strymon Big Sky to replace my defective Lexicon, but couldn't get used to it - handling and sound wise. So I sold it and bought the Specular Tempus based on some example video's I found online. For me the Specular Tempus' algorithms have a lot more instant wow factor than the Big Sky. Especially for long and lush reverbs, but I also found the "standard" (Room etc) sounding very good and instantly usable.
The SpecLab software is very easy to use. The user interface looks a bit outdated, but it's installed easily and works quick and intuitive. When using the device without the software, you can only switch banks upwards. Meaning you have to skip quite some banks before getting to the previous one. Unless you buy the 3-way switch, that is. For that reason only 4 stars for handling.
That said, as I only use this unit in the studio, and usually only have 1 or 2 presets active during a session, this is not a deal breaker for me. When performing live that might be different. All in all it's a very, very good FX unit. Highly recommended.