NEW YORK (MainStreet) — More than 3 million iPads have been sold in less than a week, but not everyone is happy with their purchase.

Several customers and news publications have voiced complaints about everything from the new iPad’s battery life to the way it connects to Wi-Fi. It remains to be seen whether any of these issues prove to be as serious as the iPhone 4’s antenna problem, but they might affect some customers’ decisions about which model to buy – or whether to buy one at all.

Here are the biggest issues that have been spotted so far:

The new iPad gets hotter than the old model.

After many customers complained that their new iPads were getting particularly hot, Consumer Reports decided to test the temperature of the device. Sure enough, the publication found that the new iPad reaches temperatures as high as 116 degrees, 13 degrees higher than the iPad 2. As Forbes points out, though, many laptops get much hotter than that.

It takes much longer to charge up.

The downside of the new retina display is that it uses up more battery power, which forced Apple to equip the iPad with a bigger battery just to maintain the same 10 hours of battery life customers had been accustomed to with the previous model. Unfortunately, the larger battery means it takes significantly longer to charge up. One writer for The Associated Press tested the new iPad and found it took nearly seven hours to fully charge, whereas the iPad 2 took just four hours and 45 minutes.

The apps take up more space.

Another downside of the new retina display is that the apps may take up much more space as they are optimized to match the high-resolution display. A few apps that have already been optimized are now as much as five times larger than before. So be warned: that 16GB iPad may fill up fast. (We should note this problem will also affect older models of the iPad.)

There are problems connecting to Wi-Fi.

Several customers have complained that the new iPad has problems connecting to Wi-Fi. One user wrote that the Wi-Fi reception was weaker on the new iPad than on their laptop, while another claimed their iPad 2 had “twice the wifi range” as the new iPad, even with the same settings.

Do you have any complaints about your new iPad? Let us know in the comments section!

Seth Fiegerman is a staff reporter for MainStreet. You can reach him by e-mail at seth.fiegerman@thestreet.com, or follow him on Twitter @sfiegerman.