By the sound of it the Wii version is basically going to be the PS2 version with 5 extra songs. The lack of online and DLC is fairly significant. Plus, if it is just like the PS2 version, there will be no Band World Tour which is a huge component of the game.
I have seen a number of Wii owners on the Rock Band forums complaining about it already. They actually had the brass to yell at Harmonix about it which cracked me up. Considering the the 360 and PS3 versions of the game have DLC and online, I think the blame could more fairly be leveled at Nintendo on this one. If Harmonix could put DLC on the Wii they would, as it has proven to be quite the source of revenue for them (over 6 million downloads and counting). But anyway I digress.
As for my RB experience, to be quite honest I haven't gotten much use out of the online part of it other than the leaderboards (which are fun to check out I must say). Otherwise I have only played online a handful of times with "TheElk" from the forums. It has worked great and been lag free on the few occasions I have played it.
With regards to the DLC, I have to admit that I was quite skeptical of it at first (I'm not a big fan of micro-transactions either). Over time though I saw songs I liked come out and I bought a few here and there. They do enhance the game a lot and add greatly to the longevity and enjoyment of the game.
I think one of the things that helps with this is that the songs integrate into all game modes just like the songs that came on the disc. They show up in Band World Tour mode in the mystery setlists, or you can add them yourself via the make a setlist concerts. This is handled far more gracefully than in GH where the downloaded songs sit off in their own area "apart" from the main game.
I've managed to download about 25 songs now (something I never thought would happen) and I am quite happy with my purchases thus far. On occasion they even bring songs out for $1 apiece instead of the usual $2, so it is nice that they aren't chained to one price point all the time.
The DLC definitely livened up a party we had last friday night as we rocked out to some Blink-182, All American Rejects, and STP amongst many others. Granted we played a lot off the disc as well, but the DLC helped to make it a more well rounded experience that touched on the musical tastes of all in our varied group of players at the party.
So yeah. Rock Band rocks!
