I finally got around to comparing all 20 or so of my harpsichord sound sources last night, and VSL's harpsichord came out the clear winner by a country mile; albeit with fewer choices than other libraries in terms of stops, registers, etc.
It is shameful how bad most of the others are -- especially the Realsamples historic libraries. I wish I hadn't wasted my money on those. They are almost completely unusable. The idea behind them is interesting and should provide a LOT of variety, but the execution is sloppy, and scripting is almost non-existant.
The next-best source is Soniccouture's recent Conservatoire Collection (which also includes a Theorbo and a Baroque Guitar). The output is insanely low, and this is coming from someone who generally records at -24 dBFS! That's my main complaint, aside from it being a somewhat limited collection (yet well-recorded and pretty good scripting). It offers a French and Flemish Harpsichord (I generally detest the latter).
Surprisingly, after last week's Version 4 update, Pianoteq is now in the race finally, but for some reason their Grimaldi emulation sounds more like a Blanchet model than the one labeled Blanchet! In some ways this is the most playable one out there, and the timbre is fairly convincing finally. But it offers no variation on 2 x 8', 8' + 4', etc. (unless I missed that on a deeper editing page).
Precisionsound's Blanchet library is also quite good, but closer listening revealed some phasing issues and inconsistencies that ultimately put it a bit behind Soniccouture and Pianoteq. It's a perfectly usable library though, if not soloed (for which purpose I will now use VSL).
Everything else trails FAR FAR behind! That includes the Realsamples collections (English, Spinet, Lute-Harpsichord, Italian, French, Dutch, Dulcitone), EWQLSO, Bolder Sounds (ancient, so no surprise, and their newer libraries are wonderful and even some of their older/smaller ones, so I wouldn't rule them out for future releases), and MachFive (from Acoustisamples, I think). Also even my earlier favourite years ago, which was MOTU Symphonic Instrument. It sounds a bit compressed and phasey compared to newer offerings.
Up until a few years back, I owned a Roland C-80 Digital Harpsichord. It was fun for a few years, but ultimately disappointing as it sounds phasey and compressed as well as a bit baked-in even when "dry", which is typical of Roland. The newer C-30 is a big improvement but I can't see spending $4000 for it, and also only one or two of its models sound good. Maybe the next rev will be "the one".
Anyway, this is all just meant to point towards why I would like to see VSL go for some of the other ancient instruments, as even the somewhat limited Harpsichord library shows that anything they come out with will be miles ahead of anything else currently available.