We use ali on all of our frames and it serves well. Codprawn is right in that the diameter has to be thicker (but not the wall thickness if designed right)which, in the case of 10m boats can look better but not so for smaller boats.
As a single frame a minimum of 50mm but we would rather 75mm. For a double frame which is our norm we use 50mm.
A lot of workshops dont like ali as it is defined as a difficult metal to weld and work. This is not the case, it is just buying the gear, getting used to it and in particular its ability to conduct heat and distort. Bending of ali pipes can be difficult depending on the grade. We use MIG welding for speed and TIG for finishing.
Corrosion is an issue as it oxidises but a frame should well do the life of the boat. The big issue comes in when you introduce s/steel fixings as this brings in electrolysis. On our consols this will cause paint blistering over a period.
Never never put ali and brass/copper together - the ali will corrode in weeks.
For tanks we consider ali to be a good value material and would price an ali tank much less that a s/s tank and it would suffer far less with hysteresis.
However correct bonding is essential to ensure that it is not the tank that is sacraficial and there is a risk area where the tank sits in the hull --- you have the grp (producing a slight alkali/some water then the ali and you will find pitting of the ali in this area at approx 50% greater rate than s/s.
You will also finf the problem with the tank that somewhere you will have another metal introduced eg. the fuel sender and this can make the ali sacraficial.
It is though a brilliant material for weight/strength and many superb boats such as SWs Ocean Dynamics will testify to that. If I remember Shaun did a good article in RIB INT last month discussing its properties.
And yes - it dont shine like s/steel !