I have been using SQLPrompt for a while and still like it.  It doesn't always work but I've learnt to work around it.  The slowness is usually just when you open up a new connection.  INSERT statements seem to produce a list of tables rather than columns of the table you are writing the statement for.  I also get times when it just stops giving me options completely. 

I wouldn't write it off completely though.  It's a great idea and some features are really useful.  If Red Gate can iron out the bugs it'll be worth paying for IMHO.