I used Play Now to get in my first online game with NHL 08 and captured it broken down into three videos by period.
Overall the game played very smooth, very close if not identical to offline. I'll need to get some more games in (including the co-op) to make any determination as to ultimately how good the performance is but that is certainly encouraging. Other EA titles over the past few months did not start off this well.
As far as I can tell there is no way to change the difficulty setting online even in unranked games. The options are gametype (ranked vs unranked), max DNF, skill level, private session (yes or no), game speed (slow vs medium vs fast), period length (5 vs 10 vs 15 vs 20), rules (NHL vs International vs custom vs off), injuries, fighting, and penalties.
Online modes include Versus Play (standard game with one on one or multiple people on a single console), Online Shootout, Online Team Play (up to 6 different 360's connected into one game for 3 vs 3), and Online Leagues.
Continue on to watch the videos. The pace was a little more frantic as would likely be expected in a user vs user game. I also noticed more hits and penalties which is probably tied to the reckless abandon with which the game was played.