IME, it gets pretty laggy beyond a few hundred svg elements. The catch to using canvas with d3 is that you lose the ability to bind event handlers to the canvas "elements" (since on a canvas they're not DOM elements, just pixels).
For my latest project I used a kineticjs canvas to get handlers on the canvas shapes. That's positioned on top of a d3-bound svg container. Best of both worlds, reasonable speed up to a couple thousand canvas shapes. Though it was quite tricky getting the same input events to trigger separate handlers on sibling dom elements. (Can't just let the events bubble up unless you make the canvas a child of the svg or vice-versa, but that approach creates more problems.)
For my latest project I used a kineticjs canvas to get handlers on the canvas shapes. That's positioned on top of a d3-bound svg container. Best of both worlds, reasonable speed up to a couple thousand canvas shapes. Though it was quite tricky getting the same input events to trigger separate handlers on sibling dom elements. (Can't just let the events bubble up unless you make the canvas a child of the svg or vice-versa, but that approach creates more problems.)