Still low in volume compared to the other two. But I do suspect the quality of traffic is at least some better than Google. Of course this will vary depending on your industry. for example I suspect a higher percentage of webmasters use Google. This implies that if your target market does not include webmasters you might do better with MSN and avoid some webmaster clicks.
MSN is the new kid on the block but is growing. Overall a good quality engine with the muscle and money of Microsoft behind it. That being said it cannot be ignored or overlooked.