This weekend there was a major tourney in germany (100+ ppl) and the winners finals was a Diddy (Luigi player) vs ICs (istudy) and I learned a lot of things about the match-up by watching/talking to the players...(istudy won the tournament in the end btw).
1.) If Diddy throws a banana upwards in the air it'll free Diddy out of a grab because it drops down on either Popo or Nana causing them to slip and thus interrupting the CG. Luigi Player is the only Diddy I've seen so far doing this but throwing the bananas upwards rather than forwards is a pretty good strategy in general.
2.) Diddy can seperate the ICs fairly well ... he has a good dashgrab, nice grab setups and quick, effective throws...he can also spike Nana but he lacks a move that can take her out quickly (like Snake fsmash) if he fails to spike her.
3.) Diddy is pretty hard to grab in general ... from what I've seen the ICs can't shieldgrab his dsmash and all his other moves are either safe on shield (banana throw) or have good enough range to be unpunished (ftilt or aerials). He can also glidetoss backwards and he is generally mobile enough to keep himself at a safe distance.
4.) Diddy has some troubles doing long strings of combos because one of the ICs will always interrupt him. ICs are the hardest character for him to deal damage to because of this and KOing is always an issue. Bananas are better used to seperate them ...
5.) I don't think that blizzards are very useful ... Diddy can just throw a banana and then seperate the ICs with a grab. Aerial ics blocks are much better if you want to predict his approaches because you can double jump away / airdodge if he throws a banana.
6.) Try to use the bananas to set up one of your grabs. This is pretty hard to do though because Diddy handles them so well and his whole charater design is based on his bananas ... ICs on the other hand...not so much =P
But it's still the best way to get a grab...
I think the match-up is 5/5