As Wi Fi is built into the DS you always have it available. Not having WiFi isn't an option with the DS.
If you mean is the game any fun without connecting to another machine then yes it's great fun. But you get more options and enhancements if you connnect to a friend who has the same game.
it is good not to have wi-fi, as it is a lot safer to not have anyone online in your town. I used to go online, and I have heard many stories from some of my freinds about people chopping all their trees down, seeding their towns, (hacking them basically, it is very bad), and abusing them through the talking option. My advice is only visit people who you know and trust. :>