When I first came to Guadalajara, I mistakenly thought that the place to see and hear mariachi bands would be in the Plaza de los Mariachis in the historic center of Guadalajara. Unfortunately, that has become rather run-down, and some even advise that you not go there after dark. When I went (in the afternoon), there was one band there taking requests, and they were not really very good. Since Guadalajara is known to this day to be the home of mariachi music, I figured it had to still be going on somewhere. Upon asking locals, I found out: it's all (or almost all) to be had at El Parián in Tlaquepaque.
Any cab driver should be able to get you there from Guadalajara, but as it's on the edge of the metro area in a neighborhood (once distant from the city, now assimilated by the urban sprawl), expect a 20 minute ride at least from the city of Guadalajara. One can also get to Tlaquepaque by bus (a much cheaper option), and locals, once you've arrived in Tlaquepaque, can point you toward El Parián.
The setup is this: the central courtyard in the middle is surrounded by several different bar/restaurant. It takes up an entire block. You choose your bar/restaurant and take a seat. If you want to experience the mariachi scene, you need to sit inside, facing the courtyard, but if you want to watch the Tlaquepaque foot traffic and perhaps hear a little mariachi music in the background, you can sit out in the outside sections. A group will perform for a while in the 'kiosko,' and then later on, after about 9 or 10 pm, individual groups will come around to your table to ask if you have any requests. Expect to pay about MXN $100 (about USD $10 at the time of this writing) for a song; that may sound expensive, but realize that you're usually talking about a band of 10-15 members.
The food and drinks are okay, but nothing to write home about. All the different bar/restaurants suffer from a typical Mexican trend: they all serve more or less the same dishes. It's not like one place serves Italian food, another traditional Mexican food, yet another Chinese and perhaps another offers licuados and bionicos. No, they all offer more or less the same traditional antojitos, which makes you wonder why they bother distinguishing the places by different names. Oh well, you don't really go for the food as much as the show anyway, but it would be nice if there was a little more variety.
Last but not least, in my experience there, the bathrooms were just short of terrifying in their lack of cleanliness and odor. That might have just been the one I went to, but again, because there seems to be not much difference in the menus of the different bars/restaurants, I just concluded that they probably all have equally hideous bathrooms. To be honest, I was so scarred by my first and only attempt to use the bathroom in the bar/restaurant where I was that I didn't have the courage to venture into any other of the bathrooms after that. If you can manage it, go in advance and if you drink a lot of cerveza, try to hold it until you get back to your home, hotel, etc.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.