It's nice, but sometimes not as nice as it seems.

Since MK is across the street from the station, it is a bit of a pain if it is raining, while the regular taxi stand on that side of the station is covered (I can't remember if the taxi stand on the north side of the station is covered; it is not well covered when taking a taxi TO the station).

On weekends during peak periods of the year (cherry blossoms and autumn foliage), we've had to wait longer than 10 minutes even though I noticed that the regular stand on the south side of the station had taxis.

So sometimes I ask myself if it is always worth the trouble to save about ¥700-800 (although at one time in heavy traffic I've seen the meter go over ¥1000). But we always use it, because free is just too tempting.

If you're staying multiple nights, you still only get the free taxi once during your stay. If I were arriving at Kyoto Station with heavy bags and thought I might be able to use the taxi service on a different day (note the hours in the Wiki), I might take a taxi from the regular stand with the bags and then use the MK service at another time.