You have to look at what constitutes a good advert. A good advert is one that gets its message across and the consumer remembers it. It doesn't matter whether whether you like the characters or not. Some of the most enjoyable adverts are actually very bad because even though they made me laugh I cannot remember what the product was at the end of it, and more to the point, don't care about learning what the product was.
The Dell advert seems to be working because it is different to the rest of the PC adverts which state "brandname" and a bland list of specifications, which to be honest, no one really cares about. The Dell advert simply states that the computer does what it can do and that it can do it, much like apple adverts messages.
I think that the reason that Steve has been chosen is a clever piece of pyschology. Computers are bought by people with at least some money (they are not cheap). This means people who are working and a lot of them are parents. Computers are an odd product in that the kids usually know more about them than the parents, so by presenting an older teenager with the message the parents are being told that this is something that their kids would use, and if the kids would use this computer then it is probably OK because they know what they are talking about. If an old guy came on the TV and told you that his computer was the one to use then you be less likely the believe him. The ad also brings computer sales down to a personal level - Steve appears to be a real person - we are talking about 'him' and the Dell adverts themselves.
The fact that Steve is really annoying just means that you are more likely the remember the message than not. This will make it a good advert - it gets it message across and you will remember it. The fact that we are talking about it in forums will have the ad agency (and Dell) very happy.
Look at the best apple adverts - they had someone doing something with their mac/ipod, etc. They could have probably had more branding to distinguish between a Mac and a PC (obvious to us, but not to new purchasers) but you ended the advert wanting to know what the product was that could do all that.
Roger.