I have now used the three dollar case from eBay for more than a couple of weeks as well as traveled with it. As mentioned previously, I purchased an iPhone 4 bumper case, and found that many of the third party chargers from my iPhone 3GS days did not fit the case. This was discouraging to say the least. Since my wife also has an iPhone 4 and has purchased 3 different silicone cases from eBay ranging from one to three dollars, I tried one out and found it to be infinitely more useful than the bumper case. Although the bumper case is more aesthetically pleasing, it had other faults as well. I noticed that HSUPA speed was significantly hampered when the case is on. I have no signal issues with the three dollar silicone case. In the case of cases, the price is certainly not a good representation of the value of the case. No puns intended!
In conclusion, if you have previous iPhone 3GS accessories, then I would steer clear away from Apple’s iPhone 4 bumper case. As for the silicone cases, just make sure it doesn’t cover the flash, so that glares from the cover does not ruin the photos that you take when you have the case on.