i think it's worth the time b/c all the cards are already made for you. i scored in the 250s and used it quite extensively. but of course i used a lot of other things to score that well [such as picmonic; also when i went through uworld, i'd spend 1.5 hrs per 10 questions b/c i was reading everything very closely and annotating all key concepts into first aid, and then i read my first aid over my dedicated study time, so brosencephalon wasn't the sole factor in me scoring well, but it was an immense aid to memorize key facts].
i would say that the pathology cards and the organ system cards are worth it.
i think it';s reasonable to do both brosencephalon and USMLE Rx. for brosencephalon, it depends how much time you want to put into it. it's ideal to keep up with the cards as long as you can, but i think it's good to use to help learn the information really well the first time. like maybe it's good to keep up with the cardio pathology cards for a month, but don't stress about keeping up with them for 8-9 months. it's really up to you. give it a try, especially with the pathology cards, and see how you like it.
i did 10 questions per day starting early november during first semester of MS2, and then 20-25/day during my second semester. a lot of people will tell you that you should "save" them so you can use them better, but i think it's actually better to start uworld early and get through it before dedicated study period. my reasoning is that the content in uworld is immensely valuable and it's key to use it as a learning tool and learn the concepts well (and integrating its information into first aid). people want to master the information before doing uworld, but you can't truly master information until you do practice questions. by the time i entered my dedicated study period, i had done the key brosencephalon decks, had gone through all the picmonic cards i needed to, and had all of uworld written into first aid. the last of those 3 strategies was key b/c then i just spent my entire dedicated study period going through my first aid which already had uworld written into it. i also did a 2nd pass of uworld during deidicated [70-90 questions a day] but i was able to go through the questions quicker.
take all this with a grain of salt. you'll get some people passionately argue that you should wait and save uworld to use later, and theyhave some legitimate points, but i think it's important to get the ball rolling now. my opinion is that the key is to integrate uworld into first aid [whether i got a question right or wrong, i'd read that corresponding part of first aid, then read the uworld explanation [really trying hard to understand concepts], and then write abbeviated versions of the uworld concepts into that corresponding part of first aid]]. it took me about 10 mins/question, but i was using that time to learn the information, rather than using uworld as a performance assessment [i don't htink you should concern yourself with your uworld %].
i didn't use rx, but know people who used both uworld and Rx throughout the year [but they started using them during day 1, so they had ample time to do both].