The predominant variety at the Mittelrhein (about 95%) is riesling; there are virtually no reds, and the wines tend toward high acid, highly floral, and profoundly mineral-imbued.
The Mittelrhein is on the big river as it plows north after a twenty mile detour going west. The canyon is steep and the vineyards are on these steep slopes, virtually all of them slate. It's a blue slate, similar to that of the Mosel. Sometimes it looks almost like pure rock. It makes you wonder what on earth possessed the early monks to think of planting vines here (though, obviously, they grew on the slopes wild. But it is a beautiful area, with ruins of castles (some converted into fancy hotels) and walled towns and just wonderful scenery. The many tourist boats of all sizes attest to this.
For quite some time, the Mittelrhein was in decline. Good vintages only came about once in every 3-4 years. The work was extremely difficult on these steep slopes, and often you had to cross the river to get to them, which meant a ferry as there are no bridges between near Bonn in the north and all the way to Mainz in the south.
But good vintages have been consistent since the late '80s and the technology has improved. Even so, there are really only a couple areas, around Bacharach and around Boppard where wines of distinction are made. It is improving elsewhere, but that's still the general lay of the land.