MagLev is only marginally faster than any of the current crop of high speed trains (HSTs) (MagLev's current record is 581 km/h compared with TGV's record of 575 km/h) and is significantly more expensive to build.
However, MagLev's ability to operate reliably and with low on-going maintenance far exceeds that of the HSTs. As the Wikipedia article notes, "with conventional railway trains, at very high speeds, the wear and tear from friction along with the concentrated pounding from wheels on rails accelerate equipment deterioration and prevent mechanically-based train systems from achieving a maglev-based train system's high level of performance and low levels of maintenance. Indeed, it was concerns about maintenance and safety that convinced Chinese authorities to announce a slowing down of all new conventional high-speed trains to 300 km/h".
Of course both high speed rail and MagLev need a track to run on, but at least MagLev is amenable to elevated construction.
The big problem with both of these technologies, and the reason they require more and more energy to go faster and faster is the air through which they travel. The quicker the vehicle moves, the greater the air resistance. Worse, the force needed to work against the air increases exponentially with the speed.