The iPad is the brand new device in the electronics world that is very popular. Could this type of device realistically replace your main computer? Is it good enough to be able to function this way? There are some very real concerns as to why this device might not be good enough to become your primary computing device. While it is something like a laptop, an iPad might not be able to do everything you need.
One of the primary arguments against using a tablet device like this as a primary machine is that the device lacks USB ports. This means that you will have issues connecting to other hardware. How do you print? How do you connect your external hardware to it? How are you going to backup your data? Realistically this is something that can't be done quite yet - at least not easily. Basically, unless we see some significant upgrades to the base iPad model, you will not be able to buy new hardware and connect to this type of device. This leaves you far less flexibility that what you get with a laptop.
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The iPad has some critical missing features that other computers have. One of these is that you aren't going to be able to watch streaming video on this device. This is because fundamental plugins like Flash are not allowed on this device. Time after time, Steve Jobs has said very directly that he is not interested in supporting Flash on the iPad and the iPhone. Because Flash is widely used, you will have to give up some functionality to use a an iPad as your main computer.
Another thing to consider is the data that you already own. Will all your music, movies, photographs and other digital media and document work on iPad? Will they all have programs that will open the hundreds of various formats that you might have? The answer is very likely no. There will be many different data types that simply will not work on this device. You will either have to change your data formats or not use some of it. This will be a huge sticking point for many users.
While it's not a good primary computer, an iPad can make a pretty good secondary device. This is something that could replace another computer in your home or a laptop. If you use it primarily for web surfing or mail, it will work well. If you use it to supplement your existing laptop or desktop, it will compliment it nicely. But if you want to replace a full fledged computer, you will probably be disappointed.