Britax SI ultra car seat - advice please

have just brought this for grannys car...only im woundering now if any of u have this seat and what its like..christopher will be using it forward facing as he is 14months.

so could u answer these for me.
whats the seat like?
is it stable ?
easy to fit? comfy for LO?
very safe ?

ialso we have the concord ultimax in our car and im woundering if itwould be worth us having the britax and giving granny the concord seat ?

i want the safest in our car if u get me.

sorry for rambling