My wife and I stayed at Seifutei last week (early March) during our two-week trip to Japan. It was hands-down the highlight of our trip. Similar review was posted on Trip Advisor (edited here due to JapaniCan's 1000 character limit).
Our room was very spacious and had terrific modern touches, while retaining a traditional look -- e.g., very pretty tatami mat flooring that was heated, an air purifier/humidifier in a small in-room rock garden nook, etc.
The private onsen was VERY BIG (could easily fit several people) and had a wonderful view of nearby woods and koi ponds below. For anyone who wants to experience Japanese onsen together with their spouse or family, or who might be reticent about being in an onsen with strangers, I HIGHLY recommend Seifutei.
The kaiseki dinner and Japanese style breakfast were both excellent, and among the best meals we had during our trip.
If you are coming to Japan, make time in your itinerary to come here!