It's basically music, and it goes down to music taste (each person has one).
The fact that they make holograms and stuff for japanese fans so they can have a waifu is pretty much irrelevant because the main focus of the program vocaloid is creating music, fan service is fan service.
Vocaloids are in simple terms a "Digital singer", in which you decide what it has to sing, how to sing it, you tune it according to your taste and things like that. You can take instrumentals and add voice to that without having to sing yourself.
The quality of the music that results from using the program depends on the ability of the user, it can vary from Justin Bieber lvl (really bad) to mozart and such... it just depends on the person using the program. Anyway, about the english song used on Letterman show I find it really bad, so it's not a good start. Actually I don't like vocaloids in english yet, i prefer them in japanese, the English versions have to improve a lot yet.
Here are a nice examples IMO of nice vocaloid/utaloid songs: (utaloid is same type of program than vocaloid, which are voice synthesizer programs, but is free, lower quality tho). The songs listed are mostly covers made by the youtuber Kyaami (which one of my favs tuners).
At least listen until the chorus, if you really can't find any potential appeal a person may have to it then you just dont enjoy music lol
Until chorus at least or it doesnt count! (Don't mind the video)
More ex of nice songs :
Ritsu (Utaloid, which is the same thing as vocaloid for common mortals)
More ritsu because is awesome
If you have time listen to this also!
Gumi
About Hatsune Miku in particular (which is the first vocaloid released, there are a lot more and there are different versions of each with have their own particular tones) I find miku goes better with electronic music, but there are other good songs of different genres, it just that it depends on the person tuning the voice.
Miku
Trance Miku song, is long but sooo good, if you have time check it!
also kawaii miku :
Some fan service
Anyway, about the vocaloid program itself, it's something like FLStudio, here you can make an idea of how it is:
In the video you can see the program.