Went to Monterrey recently for a weekend mongering excursion. Thought I'd add some of my impressions and experiences, because although there is ample info available, it comes from a limited number of sources/mongers. That said, I found much of the intel provided by these mongers (Porker, Country John, Regio) to be helpful and accurate. Keep in mind these are only opinions and impressions formed from a very brief stay.

The city of Monterrey:
I wasnt much impressed. It's not a beautiful city by any stretch, though the surrounding mountains were awesome, and if you have the time and like to hike or camp, they are worth exploring. This was my first taste of northern Mexico, and I guess I just like the culture and scenery of the south better (Oaxaca, Guerrero, Morelos, DF, Veracruz, Michoacan).

Getting there:
easy and cheap to fly from the US, easy to catch a bus from the border, and easy to drive over the border (from Nuevo Laredo it's about 2 hours).

Hotels:
They are all way too expensive. I checked in at the 5a Avenida, which set me back 420p. Rooms were clean and basic.
Changed to Plaza de Arco, which set me back 450p, and was similar quality accomodations, though a bit more spacious. Negotiate for cheaper rates, chances are you will have success: at both these places, they were asking 100p more than I paid.
Looked at the Hotel Nuevo Leon, which was asking 300p, for barely acceptable rooms that bordered on dingy, and did not appear terribly secure.
What you get for the money in terms of lodging in Monterrey is an awful value. In DF if I want to spend 400-500p, I can get a sweet hotel room with jacuzzi.
Apparently the frequent visits by gringos doing business in Monterrey has sent prices through the roof.

Mongering venues:
I found Regio's map in discuss section to be a valuable resource, and so are Porker's various posts/reports. Country John's reports, while entertaining, I felt overplayed the quality of gals to be found in Monterrey, though perhaps if I was willing to tirelessly troll through each and every strip club and court the girls there 'til closing time, I might have found some of the gems that Country John was falling for.
But me, I'm on a budget, and I have been spoiled by Thailand, so I wasn't all that thrilled with the options.
I would also note that speaking Spanish is extremely desirable as you try to navigate your way round the mongering scene in Monterrey. Especially since your best bets for GFE are going to be to get a phone number from the girl you desire and arrange for her to come to your room later on.
First afternoon I arrived, I checked out Matehuala. It has a very high sleaze factor, but if you are allright with these variables, you'll enjoy it: first, the girls are high traffic. Second, you will be having sex on a cheap metal chair, or standing up in a semi-private booth. Third, the crowd is semi-rowdy and you may be the only gringo there. I found two or three of the gals on the afternoon shift to be very attractive, and did two of them that afternoon, more or less back-to-back, and the whole experience set me back around $110US. If you want to just bend the girl over the chair and nut right away, it can be done for cheaper.
Later that evening, I strolled around and checked out Obsessions, Casa Blanca (a high-end brothel), Givenchy, TVO, Infinito, Partenon, and a handful of others. I gotta say, Monterrey has more bars, dancing clubs, and 'massage' joints per capita than anywhere else I've seen in Mexico (though I've never been to TJ). The girls I had in Matehuala were looking better than any photos I've seen posted from TJ mongers. They were slim, pretty, knew how to move, and were eager to please. Almost all the girls you are gonna see in these venues have kids. Some are better preserved than others. The only place I really liked was TVO, but there were no chicas there at that time I needed to have.
I went by Matehuala again at night, and it was very crowded, with more chicas, but none were very good-looking. I found the early shift more attractive than the night shift. YMMV
The second day I went to Matehuala at opening time, and did the two gals from the day before. At a place like this, it's better to be the first customer. It being Friday, there were more girls available, and a couple more lookers that werent working on Thurs. There is one fabulous spinner on the afternoon shift, a morena with a fantastic little ass. The other one I liked was a guera from Tampico. I dont usually get into gueras, but she has a great ass too, almost Cubana style.
Later I walked into a couple of the massage joints, which are nothing more than cheap brothels, where I saw some of the nastiest-looking whores I've ever seen peddling their wares.

In sum:
Monterrey has LOTS of mongering options, and most are reasonably priced. But you may be disappointed in the quality of girls on offer, and the ambience provided for you to do your deed.