The DC controller cuts it. You just have to get used to it. Basically you have to love the system.
I'm a HUGE fan of Dreamcast, but that controller just sucks. I use a stick or a Saturn adaptor every time I can. With games like Shenmue and Skies of Arcadia though you pretty much need the standard controller.
Saturn adaptor? I didn't know those existed. I remember there was a PS adaptor but I didn't get it. The DC controller was just fine to play racing games, DOA 2, Soul Calibur, Power Stone, Sonic Adventure, Rayman, etc, etc, etc
Never got the keyboard or the mouse either. I regret not getting the maracas.
Everyone I knew had a Saturn adaptor. The things were like...a require accessory. Mine lets you use the Saturn Twin Stick as well, which is better than the DC one, IMHO, and a lot easier to find (I have two).
For racing games the PS adaptor is best since the dead zone on a DC analog stick is like %35 of the range of the thing. Its bullshit. I have the Ferrari wheel myself, I use it for F355, Sega GT, and Test Drive: LeMans. BTW, they *really* need to make a new LeMans game. That thing was totally great.
The DC pad is very similar to the Saturn analog pad, very similar, except that its inferior in every single way (not counting the VMS). The d-pad is asymmetrical shit, the analog has a huge dead zone and slips off one's thumb too easily, it only has four face buttons instead of six, and the four face buttons are teeny tiny. The triggers are more or less the same, but the early DC triggers, the R one, would break very easily. Also, the cord comes out the bottom and the angle of things makes you have to point your thumbs straight up, which is murder after a few hours.