Research in undergrad can be a fickle thing, and getting published can be a whole lot of work.
First, let's talk about getting an authorship. To make it onto the front of the paper, there is a variety of criteria. The first author is the one who actually writes the paper, and the one presumed to be the lead person on the project. The last author is the primary investigator, basically the person providing the money and overseeing the project. The full responsibility of the paper falls to the PI, as in he/she is responsible and puts their name to lend credibility to the paper. Any authorships in between first and last are deemed to be in order of contribution. Now, on to the idea of getting on it. There are varying criteria, depending on the lab. The most fair way is whether or not you make an intellectual contribution to the paper. Basically, this means that you've helped add an idea or something that makes it in. Washing dishes, data collection/analyzation, while all critical to the paper, are not good enough to actually make it on to the front. Techs can do this job, and techs dont get their names on the paper because they need to add some brain work to this. However, this standard can vary due to office politics and the generosity of the PI. Basically, people have been known to get on papers just for data collection and analysis. Also, the you scratch my back I scratch your back theory applies here. As a research investigator, the more your research papers are cited, and the more your articles you help author, the more prestige you get. Think of it as a video game, where you get points every time you appear. So basically, PI's will get some of their colleagues to read the papers and whether or not they have anything to add their names will get put on. While this might not seem fair, bear in mind these minds are some of the finest in the world on the subject, and their input can lead to truly valuable insights. So it's not all about politics, but it can influence it.
As for the time required, it varies from lab to lab. I myself did 20-30 hours a week in the heavy times, but of course it depends on the nature of what you want to do and how strict they are about doing it.
Impact factors. Basically, a rating of how "important" a journal is. It's a rough proxy of how many times an article in the journal is cited by other papers, the theory being that hey if your work is important people will build their work on it. There are the most prestigious ones, such as Science and Nature, and as someone else posted, these can be affected by what's currently "hot" in science. These two have an impact factor of about 30-35. There are the top tiers which fall between 10-20, which includes PNAS (Proceedings of the National Academy of the Sciences), Cancer, Circulation, etc. These are all very good journals as well. Other good journals like the American Journal of Physiology rate around a 4, and lots of other journals with lower impact factors. Keep in mind, these are peer reviewed, and just because the paper isn't in a journal with a big impact factor doesn't lessen the importance of the research or the experience. It depends on the field, the scope of the journal, and a whole bunch of other things.
Now all this goes into the prestige points the investigators can earn. A new rating system of the approximate 'importance' of the researchers is how many times their papers have been cited by other papers. It's a weird system, I wont get into that. Authorships are pretty important, because the grad students need them, the postdocs need them, and the PI's need them for a variety of reasons. Anyway, authorships are hard to come by as an undergrad, because even grad students can toil for years without an authorship (first or otherwise). A little luck, a lot of hard work, and lots of open communication with your supervisors will get you there. First authorship might be too hard for an undergraduate, because like I said, grad students who do this FULL TIME take YEARS to get to that point. It's not likely there's a project just waiting for you right there to take.
Back to the original question. Research is definitely worth it, even if you don't get an authorship. The intellectual benefits and the experiences have definitely proved a huge plus for me. Working with people who are absolutely amazingly intelligent, getting projects and contributing intellectually, it's great. Like all things in life, you get out what you put into it. The more you put in, the more you get out. An authorship will impress research oriented schools (all the top tiers), but you need the whole package with them. Patient care oriented schools don't care as much about research, but it's still a nice feather in your cap. An authorship says "Hey, my PI/supervisor thought enough of my contributions to give me an authorship!", which hopefully still means something nowadays. Even if you don't get an authorship, you can still put the experience down on your AMCAS app and talk about it in the essays that they have for you. You can talk about it at interviews and stuff.
I apologize if this was a long rambling thing. I'm quite sleepy.
Addendum:
This does not apply to "undergraduate journals". I don't hold much stock in them, as I don't know anyone who reads them besides people who are in them. The peer reviewed journals are the way to go. The Ugrad ones are 'peer reviewed' by students, who probably have no idea what you're talking about in the article. Adcoms might feel the same way, or might be impressed you bothered to try to write up an article type thing. Also, this all applies to basic science research, I personally have no clue about clinical research and how it works.
