What to expect?
As you can see from my answer to "recommending vdlp", me and my husband experienced full albergues several times from Sevilla to Caseres in April/May 2006. During the walking we often saw nobody else for the whole day. But at the end of the day we all gathered in the same villages/towns. Thats one of the big differences between VdlP and the french route. The peregrinos are not spred out on many villages and albergues. The stretches between villages are much longer. And there are not an albergue in everyone. From different reports it seems like you must be prepared for being alone, AND finding no free bed in spring. But there are hostals.
When so many report that VdlP is harder to walk, I think there are different reasons for that. The distances between the villages are longer. Therefor there are less posibilities to f. ex. take a rest on a bar during the day. Often it is also difficult to find a shade for a rest. You also have less posibilities to decide how long distances you want to walk. May be the next village just must be the place to stop for the night whatever distance. Also: You meet less people. The temerature is higher earlier in spring than on the french route. Sometimes the landscape is "hard". For example, I have never before walked almost 20 kilometers stright ahead with only wine and olivetrees, olivetrees and wine. Not even on the Meseta between Burgos and Leon. I found it relaxing, but others have called it boring.
The marking is good. And we had our "tostada" and "cafe con leche" on a bar almost every morning. But I have heard that there are less posibilities further north. Different people will have different experiences both on VdlP and on the french route, but for me one of the big differences was what I will call "room for spirituality", both as part of the Camino and with the people walking. There were less posibilities to go to mass. Often there were none, other times we could not find a person or a poster who could tell us where and when. Among the pilgrims we found more what I would call "long-distance-walkers". We met many nice people and had a lot of good conversations, but did not find the "room for spirituality" as we have found it earlier years on the french route. I missed it. (But may be other pilgrims can give you other experiences! Also be aware that we only walked the first 300 km.)
So: Harder, yes. Different in many ways, yes. But to recommend, yes! We will be back to continue another year, and we will choose April ones more (may be a little earlier in April). The temperature is good, the nature is green, but most of all we choose April then because of the flowers. I have never seen anything like it in my whole life. Bjørg