RIFT will gain its own little niche of players. I think it will be a nice contribution to the MMORPG community.
Will RIFT be a success? IMHO yes it will be. Will it compare to WoW? Unfortunately not =/.
I try not to think about upcoming mmo's as "WoW killers". Wow has become essentially the mmo paradigm, to the point that "mmo" and "WoW" are practically synonymous in some peoples minds. But there's always room for new games, especially when said games are trying something different. Rift won't topple WoW off it's throne or anything, but I predict it'll have a decent following and a good run, provided the devs implement everything well.