Finally got around to setting up my 54120B on the bench, first time using this scope, this is what I got up and running about five minutes from switching on. I'm using a 54121A TDR test set with a short on the TDR/Ch1 channel for the pulse. Total of $700 of TE from ebay, in three separate auctions for the 54120B, 54121A and interconnecting cable, the cable cost almost as much as the 54120B.
Nice scope, which brings back fond memories of using them when they were the dog's bollocks, and of chatting with the digitiser's creator. They are still useful on this forum, since they are a clear demonstration that the "samples/s" spec is completely different to the "front end MHz" spec
Unfortunately, for directly measuring eye diagrams of many of today's high speed interfaces such as USB 3.x and HDMI, I find the 54120B and its additional modules less than perfect.
The problem is that the p-p voltages are fairly low, and typically you need to split off the signal you're measuring into both a scope channel and the trigger input. If your signal has a longer term DC component in it (or is bursty) then you need to use a resistive power splitter to maintain DC adding to your probe loss, so that's typically 26dB down in total with a typical x10 probe. When the single ended p-p is only 300mV or so, that doesn't leave much for the scope's channel front end.
If you use a directional coupler to reduce losses into the channel input, with the trigger sampling off the coupled port, then that improves things a bit but only if the signal has no DC component: the coupled port will be AC coupled so this will show up as jitter on signals with much DC component.
At least with HDMI there is a clock/10 signal, so using that to trigger is usually a reasonable option but it makes casual browser probing much harder as you're almost certainly going to have to solder in a tap to make it work logistically.
Still, the biggest problem is that the scope front end lacks sensitivity for a lot practical use cases on many of today's low level high speed serial signals. Sure, you could add a broadband amplifier block, but that in itself introduces its own problems, not least that you're no longer measuring the actual signal.