First of all, they are very convenient. You can cook daals and meat in a matter of a few whistles (seeTiyan, plural of seeTi in Urdu/Hindi). Whistles? Why do Indian English language cookbooks and recipes refer to whistles? "Cook for 3 whistles." What the heck is a whistle? Well, to know you would have to watch a traditional pressure cooker in action. You add the food and water, you seal the lid, and turn on the flame. The pressure builds, then releases. To release the pressure, steam comes out of the pressure cooker's whistle (regulator). Two to three whistles for most daals, 5 whistles for goat or beef. Like so. I have an Indian pressure cooker. It is traditional. But I have heard that in Amreeka you can get modren pressure cookers that have no seetis. When I relocate to Amreeka, I will have to get one. For now, I am stuck on my seetis. So, you know how you have to simmer beef for 6-8 hours to get it nicely tender? You can pressure cook it for 5 whistles and in about 20 minutes your beef is done. Cooking fuel is expensive in the des for average families, so people save money on cooking gas by using pressure cookers to reduce simmering time. Pressure cookers are economically sound, and good for the environment since they lessen fuel consumption.
There are some downsides to pressure cookers. I love the way large legumes like chickpeas, rajma, and black chickpeas come out in the pressure cooker. They get such a tender texture that allows these legumes to grab on to flavors so well. However, I don't like the end result of how small sized daal comes out. I mean the types of daal that are cooked to a liquidy consistency and are not meant to remain whole. Somehow the pressure cooker makes the liquidy daal fluffy. I don't like the texture. So in my kitchen, I only pressure cook large sized daals which are supposed to remain whole. Secondly, though it is a great meat tenderizer, for me it ruins the taste and texture of beef and goat. The meat also comes out puffy and fluffy. It isn't horrible or anything, but I can always detect pressure cooked meat, and prefer slow simmered meat. I do occasionally resort to pressure cooking red meats as a time saver only. Chicken and fish are a no-no for the pressure cooker, as they would just be obliterated, unless the chicken where you live is quite tough. I do use the pressure cooker to tenderize chicken in specific dishes, though.
After the pressure cooker has done its job, you still have to wait for the pressure created vacuum seal of the lid to release or "fall in." This adds to your cooking time. Some people advise running the pressure cooker under cold water to release the lid. But actually, waiting for the lid to fall in is part of the cooking time of many lentils, so if you release the lid early, the lentils may not be fully cooked in the center. So it is best to wait for the lid to fall in on its own.
The pressure cooker is a necessity in "mehnati" or arduous dishes, like daal makhani. Without the pressure cooker, one would have to babysit this slow simmering daal for 4-5 hours, stirring and mashing away, and adding more and more water as needed. But the pressure cooker reduces your cooking time to 15 minutes of allowing the pot to boil, then waiting for the whistles, and another 10 minutes or so of waiting for the lid. You can't beat that.
I also make stocks in the pressure cooker. For chicken stock that would normally take over an hour, I allow the magic of the pressure cooker to pressurize all of the great flavor out of my stock cuts in around 15 minutes.
You can also use the pressure cooker for some special dishes. For a Lahori style chargha (marinated, cooked, then deep fried Lahore style crispy chicken---recipe coming soon), you can tenderize the chicken so beautifully by giving it one whistle in the pressure cooker, allowing it to cool, and then deep frying. This yields absolutely amazing results.
I know some people who use the pressure cooker for everything. Everything! A friend once told me that people are so stuck on pressure cookers in her part of India, that even their pudding (payasam---South Indian kheer) is pink because apparently milk turns pink in the pressure cooker. Many desi cooks would be lost without it.
Everyone whose family uses pressure cookers had stories of a forgetful auntie who left the pressure cooker unattended and the dang thing exploded, leaving tamaatar saalan stains on the ceiling. So, one should learn from these stories to never leave the pressure cooker unattended and to pay attention to the whistles.
Another note on the small daals: sometimes as they liquify, a tiny piece gets stuck where the steam is supposed to come out, blocking it. This will cause your steam release to spew an ugly yellow daal mess out of the whistle. I have only ever had this happen with daals, but theoretically, any small piece of something could cause the blockage.
I can't wait to use the new modern pressure cookers with their safety features and double seals. But for now, I stick to my old Hawkins quite happily. I advise all people who regularly cook desi foods to invest in one! Luckily mine was cheap over here in Dubai (and it came with a free idli tray!) but I have heard that they can be costly further away from the des.