The only reason to observe the venerable Ashkenazic prohibition on kitniyot/legumes is to maintain one’s own family tradition. Which is a perfectly adequate reason! If you wish to forego beans and peas and corn and rice because that’s how you and your family have always kept Passover, then more power to you.
But if you do not have a particular family tradition — for instance, if you are newly observant or a Jew by choice — or are vegetarian and need a wider variety of menu items, then I think it is fine to consume kitniyot. Actually, it is more than fine: eating kitniyot would remind us that true hametz can only be made from wheat, barley, spelt, rye, and oats. That is the core prohibition. Let’s not to make the stringency more important than the actual rule. This is my own practice, thanks to my vegetarian children.
The Rabbinical Assembly Guide gives much helpful direction about purchasing and using kitniyot. Let me add a couple of points. In recent years, the number of kitniyot on the market with Passover certification has grown tremendously. In kosher supermarkets in Queens, Brooklyn, and New Jersey you can easily find rice, beans, peas, canned corn, hummus, techina, and other products certified for Passover. Some of these might be marked in English as “only for those who consume kitniyot” or in Hebrew לאוכלי קטניות בלבד. That is what you want to see and can feel confident. For rice, lentils, beans and the like, you can also purchase non-certified products before Passover, pour them from their bags into other containers, visually inspect the contents, and discard any stray matter you might find. That is perfectly adequate.
People have asked me about soy products. I follow the practice of the Sefardic West Coast Rabbinical Court (students of Rabbi Ovadya Yosef) which rules that plain tofu with kosher certification for year-round use can be used for Passover without special certification. You should buy this product before Passover begins. This Beit Din applies this rule to Tofutti vegan sour cream and vegan cream cheese. They apply the same rule to unflavored soy milk and rice milk. You can buy these products before Passover on the strength of a year-round hekhsher. Do not use flavored versions, which can contain actual hametz.
People have also asked about miso, which is a fermented soy paste. Unfortunately, the same rule cannot apply. Different misos use different agents in the fermentation process. Yellow and red miso typically is fermented with barley and therefore is actual hametz. Do not use miso without Passover certification. (Unless you made it yourself.)