Short Answer: It depends.
Longer Answer: It depends on your situation.
Very Long Answer:
Depending on the plan you may either get a free PC or about $400 off a PC.
I think MSN, Prodigy, AOL, etc. are giving $400 rebates if you promise to sign up for 3 years or so. Usually you can buy the PC at BestBuy or CompUSA or some other such retailer and when you sign up you get the rebate.
Other deals like peoplepc, give you a couple of choices of computer and it is free if you take the base options.
Now, you get a lot off, but what do you gain?
The PeoplePC deal is about $24 per month for 3 years. That works out to be about $864, but since you would likely have had to pay $15 - $20 per month anyway, it only costs you about $150 - $300 more than without and you get a free computer. The other companies, I do not believe charge much if any extra.
However, if you take the PeoplePC deal, you get a 366 Mhz or so CPU, a 3 GB or so hard drive about 32 MB of RAM, and a 15" monitor. You can upgrade to a 17" for $99.
If you go with another offer that is $400 off and you want a PC and you buy a better PC then you may not end up with an underpowered PC.
Now, currently a 366 is not really underpowered, but in 3 years it will be a dog. For example, 3 years ago, I bought a 166 Mhz computer, they were coming out with the 200 Mhz (not even the MMX), but the 166 was about the fastest. Now, most people would scoff at a 166. So keep in mind that what is ok today (and the 366 is ok at best), will not likely be ok tomorrow.
You definitely do not want to play any games on it. Internet and e-mail and word processing should be fine though.
Hope this helps.