No, Dell has not decreased in quality at all. If anything, they have become more vigilant toward what they are putting into their computers, giving their customers a much better experience overall. Again, this is going to go towards personal experience but having used many different brands of computers myself, Dell has been among one of the best that I have bought.
What you pay for, you will most definitely enjoy the computer that you end up with. Many of their computers are able to be customized and this gives the customer a much more hands on experience and a computer that they will love for years to come.