I used the Dream World a bit; I actually got a 3DS just so I could get online in Gen V (granted, it was mostly to get hold of the 2014 Movie Victini, but it still applies), because I couldn't do it with my DS Lite. The thing was, I found it really difficult to get online, even with my 3DS; for the longest time, neither Mystery Gift, the Global Link or the GTS would work for me. I still can't stand to listen to the Gen V version of the Global Link music because it reminds me of all the failed attempts make the thing work.
Once I finally managed to get onto the Global Link, I found it frankly a bit lacklustre. It just seemed to take too long to be able to access different areas and unlock new Pokémon to meet, at least for me. On the other hand, being able to get hold of Hidden Ability Pokémon was cool; I ended up getting hold of a female HA Aerodactyl once, so if I want to breed a competitive one, I'll be able to get it in a Dream Ball.
The Dream World was a good idea, don't get me wrong; I just think it could have been implemented better. Maybe fixing the long load times, or maybe offering more Pokémon like the event Lucario that was available at one time? I don't know.
I also have to admit that I'm a little salty about the Dream World for a reason related to my numerous failed attempts to access it: does anyone remember the popularity poll, where eventually an Arceus was given away over the Global Link? I entered the password for it on the Global Link long before the end of the event period, but couldn't connect to the Dream World to pick it up for some time. By the time I was finally able to access it, Arceus had somehow vanished. The one time I could have got an Arceus, and the universe conspired against me to take it away.