I have to count myself among the dissenting on this as I mentioned in a couple of other threads. Too many strikes against it for me to believe it will work in the long run. It might be popular during the free period and when 4e first releases but I think it will wane after that.
1. If I don't like what's in Dragon/Dungeon on a newsstand I can skip that issue for the month; no such luck with this option. Subscriptions for both magazines for a year weren't even that high if I remember correctly as they tended to be discounted by 35-40% except for outside the U.S. to which then yes it's a good deal.
2. The miniatures are not included in the expense. If they include the actual miniature in the cost of buying the online version it would be worth it to pay for them online as I'd know what I were getting versus buying mixed boxes. Need 20 orcs, skeletons, whatever then I buy them for both online and at home.
3. Content updates have been decent, if not exceptional, but will the consistency hold up? What if something happens at the company that causes a delay in content? Will we be reimbursed for the lack of it?
4. Access is subject to a number of things, i.e. server maintenance, band width usage, your computer's capabilities, hacker attacks (it can happen and when it does it causes the first one), availability to the internet where I play, and various other things I can't think of at the moment.
5. Personal time availability may not justify the cost either. I play only once a month or so it's too expensive, but if I know I'll use it more than once per week it might justify the expense. Will all that play time be online? Will I have access to the internet where I play?
6. The prices are similar to an MMO, which offers more in the sense, that if I can't get together with a group I can still play on my own. For younger players who would be relying upon their parents to pay the fee it's another justification they have to make and a parent will most likely look at the expense of two online things as starting to get expensive and would probably draw a line. Either the MMO or DDI, sorry but the MMO will win.
7. Playing with groups outside my own requires them to accept me. This acceptance is subject to their decision, any possible reputation I might have or don't have. I doubt WotC will force people to let someone join a group. I also run the risk of playing with people I don't want to play with and most prefer not to pay to play with players they don't like. Also players I don't like can change their names and join up again (I know this would require some work on their part, but irl I see the person and know if it’s someone I don’t want to play with).
8. If I buy all the books I am paying twice for them. Why am I only gaining access to what I purchase? I'd think if anything, buying a book would entitle me to a free month, which would encourage sales of products. They could also build the cost into the products I am purchasing; a small price increase of fifty cents to a dollar going toward web maintenance would go unnoticed. The $120, if I do a year subscription would buy at least four more books that I actually would use. Where I might not use the stuff anyway as there was a reason why I didn't buy it to begin with.
9. I still think advertising would be the best way for WotC to make money off of the service and just keep it free. Major companies in computer gaming would pay big money to advertise, not to mention soda companies and such.
10. As for the extra features of DDI, such as gaming table, miniatures, and other stuff not mentioned, a good retail program for a quarter or half the price would work, minus the online play ability but that is only a step away. What if someone comes up with a similar idea and does it for less or free? People have been playing online with tools in chat rooms and other methods for a long time. WotC can't copy right or patent the idea for online table gaming as it has been around for a long time already and I can't do that with ideas any way, only the way it’s done or presented. (An example for this is one of the jelly companies has a patent on the peanut butter and jelly sandwich, not on the actual idea of that kind of sandwich, but on how they make their specific sandwich to be sold.) If another company decides to make a generic program which could be tied directly to D&D by use of its tools then that competition would mean the price needs to go down or go away depending on how much of an impact said product might cause.