It is pretty much the same as a bank, but the ownership is structured differently.  BSs are still regulated by the FSA as are banks and are insured up to £35k as in a bank.

Treat them the same.  You may find that some of the local Buiding Societies have restrictions on accounts, most common being you have to live in the area.  Check the account details.

Looking at the interest rate, it is similar to the highest paying ISAs.  Have you used your ISA allowance for the year?  If not, you would be better using this up first as the WB account would be taxable. 

Hope this helps...