Ciudad del Este, the Paraguayan city across from Foz do Iguaçu is known for one thing and one thing only: it's a black market haven, especially electronic goods. I didn't buy anything myself but I can confirm this - my friend Lucas bought a high-end camera for a third if the price he'd have to pay back home in São Paulo. Legend has it that you can buy anything from flip flops to AK-47s in Ciudad del Este, if you happen to know the right people. It has a reputation for being a dangerous area so I didn't linger there long, maybe 45 minutes to take a peek (another unusual thing I just HAD to do), then walked straight back to the Brazilian side. I'm adventurous but also careful, caution is the better part of valor and all that.
Overcrowded city, I felt unsafe, prices were not actually good as expected, and the city is extremely dirty!
Overall, I would say I found Ciudad Del Este to have a bit of an unsavory feel to it. If you are a fan of buying pirated items, or a cheap car of disreputable origins Ciudad Del Este may be your type of city. Outside of the Itaipu Dam, I would recommend skipping Ciudad del Este and heading into Paraguay and exploring the abandoned Missions or going to Asuncion
There is no reason to be here. Brazilians and Argentinians cross this loose border for electronics, blankets, fake handbags, and the sort. For the average tourist, there's really no need to come here. I've heard from many people that walking over the bridge that links this border (specifically between Brazil and Paraguay) is dangerous and you should wait for the bus. I have crossed it 4 times on foot and once by bus, and I couldn't have felt more safe. If anything it's the only tourist kick you get when you can stop and take pictures of both Paraguay and Brazil. They even have a yellow border painted on the walkway with Brazil and Paraguay painted on each side. What I also found amusing is that when you walk over (the average tourist) you're looking for a stamp. Well you can simply walk over without any border inspection, exit, or entrance stamp. Now this could potentially give you trouble if you plan on exiting via Argentina. (Though it's easily remedied by going into an immigration office in Asuncion for free). But if you are only going for kicks and will not be leaving Brazil you can get a "souvenir stamp". You only get one, not for exit/entry, but one, with no date. The city itself, hot mess. The border was the only point of interest for me.