Hi, Sharon,
I walked this exact route last May and think it's a wow-zer. I've posted my stages and general comments for both the Madrid and the Invierno here on the forum, they are very different from each other, but have one thing in common -- they are very solitary caminos.
The Madrid, leaving right from the center of town, gets you out into rural Spain pretty quickly. Albergues are excellent, with an occasional need to go to a pension/hotel. Generally flat, except for the day from Cercedilla to Segovia, which is amazing and tracks the Roman Road with a couple of roman bridges thrown in. I really enjoyed a rest day/detour to Valladolid.
Arriving in Sahagun is a shocker with all the pilgrim traffic. Rebekah and the Peaceable are close by and good for some R&R. I was hoping to meet people on this stretch who would be interested in walking the Invierno with me, but of course by then people had formed a lot of bonds and friendships and were unwilling to break off. I have to admit it was hard for me to leave at Ponferrada because in those 5 or 6 days I had made some pilgrim friends whose company I enjoyed very much. But I did it, and I am very glad I did, even though I never saw another pilgrim until the Invierno merges with the Via de la Plata outside Lalin in A Laxe.
The Invierno is very beautiful. Well marked, and with Rebekah's online CSJ guide it's easy to follow. Unlike the Madrid, though, there is some elevation gain. A couple of hard days one after the other. Very few albergues (my post and Reb's guide give all the details), but affordable private accommodations in all the main towns. Maricristina of this forum and her husband, both in their 70s, just walked it and told me they really loved it. They took twelve days, I did it in a few less, so there are a varitey of ways to plan your stages. There were many opportunities to talk with the people who make their lives in these very rural parts of Spain and their warmth and kindness made up for the absence of pilgrims. I will admit, a few of those afternoons all alone in an albergue in a town of 100 or so were hard. But I wouldn't hesitate to walk these caminos again. Buen camino to you, Laurie