I main ROB now.
When we first unlocked him (we didn't know about the 35 character roster as we had been avoiding the internet knowing we were getting an import copy and didn't want to spoil anything) I was surprised and dissapointed at first. However, after we played around with him, I thought they did a good job animating him in a way that gave him a robotic personality and he seemed pretty fun to watch (his side step is hilarious). His moveset turned out to be pretty entertaining too.
So, after cycling between about 8 characters, with ROB being one of them, trying to decide who I felt most comfortable with, I began to notice that ROB was gelling very well with my playstyle, and I won more often with him than most other characters (second most common was Olimar, but eventually I decided he was just a bit too complex for me to main this early and I hated being edge hogged when playing as him).
Thus I've been maining ROB for the past 4 days and plan to continue doing so, with Mario and Luigi as secondaries.