I have tried Wilton, CK, and Americolor. I used the Wilton for a long time and thought it was fine until I tried CK - WOW what a difference in taste! I always thought Wilton smelled awful in its raw form but since it tasted OK I never worried about it. CK and Americolor both smell good in powder form and both taste way better. I have only used Americolor in one batch so far but I was impressed with the results.
I haven't had trouble with grittiness due to meringue powder but have experienced differences in the texture due to powdered sugar brand.
Note - I follow SugarBelle's advice and use a LOT of flavoring in my RI. Like 2 tbs - makes a world of difference