Hi there Dominic,
I have not travelled theer in summer, I went through there last year in November, also coming from Senegal. I crossed all of Africa, going through 27 countries and must say that Guinea definitely rates high up there among the nicer ones to travel through. Main reason for that is teh freindliness of the people, the naturl beauty and the fact that there are virtually no other travellers around.
I think coming from Senegal you will have difficulty during the summer months. I came from Kedougou and took the direct route to Mali(ville) on top of the Fouta Djallon. This road was dry, completely, but very difficult for me, the first stretch just after the border, you have to go up a very steep escarpment, usually only done by 4x4 trucks. There are lots of rivers and valleys to cros, so I imgine that this will be very tough during the rains. The other route, further west, is flatter, but will probably be very very muddy. My advice would be to go, if you have time, as in Africa you will always be able to "make a plan". Donflatter't go with own transport that time of teh year, will be very tough I think.
Have lots of fun !!!