I did most of this trip, from Niamey and north through Niger and Algerie to Tunisia in 2007. The driving up to Agadez is not so difficult, but its long and the road conditions can be a challange. In the north you will not see much people in between citys. Be aware of the few crazy bussdrivers, they might come a lot faster than you expect.
From Agadez up to Arlit is ok, but hardly any people around, and the traffic is scarce. From there across the borders to Alegeria, a litle more difficult. No road, only tracks in many directions. Head west out of the mining town, and look for the route marking. From the Niger border to Algerian border there is no road either, so bring a compass or a gps.
I guess you have a guide waiting for you at the Algerian border? There is a new road after the bordertown in Algerie, when I was there it lasted for about 100 k. After that it was dessert wit many possible routes going all the way to Tamanrasset. Bring food, water, snacks and basic medisin (eyedrops with and without pencilin) If you dont need it, locals will
Sahara is a big and beautiful :-)
Med mc rundt Middelhavet