I usually use the TGV because I fly into terminal 2C which is quite near the TGV station, I don't mind walking a little to the airport station or from Marne-la-Vallée station to my hotel, and I can usually time things well enough to arrive in line with TGV departures. And it can't get stuck in traffic. It does need a lot of things to line up but when it does it's fantastic. You don't need to worry about or pay supplements for child seats, or that your driver won't speak English, or that your kid will be sick. And it's usually way cheaper than taxis/private transfer and moderately cheaper than the Magic Shuttle.
Aside from the walking as already mentioned, other drawbacks of the TGV are that if you book in advance to get the best fares and your flight is late, you may have quite a wait and/or an argument with the SNCF agent about amending your ticket. If your flight is to terminal 1, 2G, or 3 you will need to get an inter-terminal bus or train to the TGV station which will erode time savings. And actually getting a TGV ticket from CDG to MLV isn't straightforward as the SNCF doesn't want to sell them because they take up seats on a long-distance train.
Someone will probably be along shortly to point out how horrid the TGV is and how you should never use it, but I think the above is balanced.
