I was on the Grand in July of this year and we did personal choice and absolutely loved it. We ate in the Horizon Buffet dining room most of the time as their selection was always quite varied and was able to please me (very picky) and my husband (loves everything).
We did go to the dining room a few times (they had 2 assigned for personal choice cruisers) and had minimal waits - I think one was 15 minutes was the most we waited. But - it really depends on the time you go and the other cruisers. If you go at a prime dinner time you may have a bit more of a wait, but you can certainly put in for the reservation and then take a stroll around some of the shops.
If you want to try one of the alternative venues (Sabatinis or Painted Desert) the availability may also vary. We were able to get into Painted Desert without much difficulty, but I hear that Sabatinis was booked. You will want to book these as soon as possible if you want a specific time or date.
Hope that this helps. If you have other questions, feel free to email me.