I spoke with Brian who worked as a roofer for a few years and he said that you're better off having an experienced roofer install it since there's not much room for error. He wasn't 100% sure but he said you use either tar or some other type of adhesive to get it to stick and if you make a mistake you only have a few minutes to go back over it and fix it. Of course you could fix it afterwards but I guess from what he says it's a big pain. He's never installed one but his brother in-law(Who owns his own roofing company) has and often turns down jobs with rubber roofing, because it's that much of a pain. **shrugs**
He wasn't sure of the cost of it vs. shingles but he didn't think it was too cheap.
Also he said that generaly rubber roofing is put on a flat roof not a steep or slope roof. I'm not sure what type of roof your parents have but he said your traditional slope roofs look like crap in rubber.
He's not an expert on it but I thought that maybe his little imput could help.