I speak both Spanish and Portuguese. I will say they are not mutually intelligible if a person has not studied either language. Even if you have studied it, there are enough differences that it would take a few years to know the other language completely. They're definitely very similar, but you have to pay attention to the way you pronounce thiings. Brazilians do not like it when people speak "Portunhol." a mix of Spanish-Portuguese. They're very strict with the way their language sounds, so I assure you that Brazilians will not allow you to speak Portuguese if you don't intend on doing it correctly. They will ask to speak English or Spanish.
That being said, it's easier for a Brazilian to learn Spanish, because Spanish-speakers pronounce things the way it is written, while Brazilians do not pronounce words the way it is written.
Basically, in Brazilian Portuguese the pronunciations would be like this:
"Onde" is pronounced, "on-ji"
"último" is pronounced "ooh-chi-mo" (the "ooh" sound doesn't exist in Spanish, it's like an elongated "u" sound")
"loja" is pronounced "lo-zha" (this sound does not exist in Spanish either, it's like the "s" in the English word "pleasure")
"musica" is pronounced "mu-zi-ca" (not mu-si-ca)
You eventually memorize all of the rules of pronunciation just like in any other language.
Also the vowels in Brazilian Portuguese are said in a way that's a little different from Spanish. You have to learn that as well, otherwise, people will say you're speaking "portunhol."
You should be able to understand writings, with about a 60% comprehension level, which is not enough to understand anything too specifically, but you get the general idea of what is being spoken of. I would recommend any Spanish-speaker to study Portuguese, if they wanted to learn a different language. It is definitely the easiest language to learn. If you study frequently, in about 6 months you should be able to have a basic understanding of Portuguese and be able to understand people when they say something. You'll be able to "hear" the language. That takes a while to do, because the moment you can "hear" the individual words, instead of just hearing mumbo-jumbo, then you know you're gaining some mastery over the language you are studying.