I highly recommend this article by Alon Levy:
Some choice quotes:
"The barf ride that is as expensive as California HSR and takes as long door-to-door is also very low-capacity. The capsules are inexplicably very short, with 28 passengers per capsule. The proposed headway is 30 seconds, for 3,360 passengers per direction per hour. A freeway lane can do better: about 2,000 vehicles, with an average intercity car occupancy of 2. HSR can do 12,000 passengers per direction per hour: 12 trains per hour is possible, and each train can easily fit 1,000 people (the Tokaido Shinkansen tops at 14 tph and 1,323 passengers per train).
But even 30 seconds appears well beyond the limit of emergency braking. It’s common in gadgetbahn to propose extremely tight headways, presuming computerized control allowing vehicles to behave as if they’re connected by a rod. Personal rapid transit proponents argue the same. In reality, such systems have been a subject of research for train control for quite a while now, with no positive results so far. Safety today still means safe stopping distances."
"There is no redeeming feature of Hyperloop. Small things can possibly be fixed; the cost problems, the locations of the stations, and the passenger comfort issues given cost constraints can’t. Industry insiders with ties to other speculative proposals meant to replace conventional rail, such as maglev, are in fact skeptical of Hyperloop’s promises of perfect safety."
"There already exists a mode of transportation that involves security theater, travel at 1,000 km/h, poor comfort, and motion sickness."
Levy actually has some more recent posts that moderate some of the criticism, but don't totally eliminate it. His most recent conclusions seem to be that unconventional modes like Hperloop are really nost practical for 1,000 km corridors, but that the costs are still pretty high for a newly developed technology.
". I for one will keep putting vactrains in my 22nd-century science fiction, but not in my near-future science fiction.."