Hi Stuart,
I was certified in January of this year in Puerto Vallarta, and have just completed my 12th dive, so I'm still relatively new to both the sport and the area. As I have nothing to compare it to, it's hard to say if it completely sucks or if it just has its moments. I have found the visibility to be both poor and excellent, but the excellent visibility will come with a price in temperature. The thermoclines are trippy. The temperature just shoots down at about 50-60 feet; we were down to 60 degrees on one dive with no hoods or gloves -- BRRRR. But then you can clearly see the pinnacles and canyons, and the deep blue abyss below -- that's pretty cool. A 7mm wetsuit, hood, and gloves is just about perfect for this time of year.
As far as marine life goes, on my certification dives I saw (and held, guilty and gleefully at the prompting of the instructor) an octopus, a seahorse, a pufferfish, some kind of spider crab, a sea urchin, a sea cucumber, and a starfish, and there were plenty of angelfish, parrotfish, pufferfish, and other little tropical fishies toward the surface. It took a few more dives to see green sea turtles, round rays, eagle rays, garden eels, giant moray eels, nudibranchs, stonefish (no touchy!), some anemones, little corals, bioluminescent plankton, and massive underwater rock formations, big as buildings. I have definitely been on a couple disappointing dives as far as the sights go, but for me, any diving is great diving...practice makes perfect!
I would highly recommend you check out Banderas SCUBA Republic, located in Emiliano Zapata, or Old Town, or the Romantic Zone, whatever you know it by. Cesar Ortega is the owner and instructor/dive master of this outfit, and the man knows his stuff. In addition to his warm, friendly attitude and obvious love for diving, he really knows the area dive sites. He can tell you whatever you want to know about a particular dive site, and tailor the trip to your interests. He's been in PV for 8 years, so his knowledge of the entire bay is pretty extensive. He does certifications through at least dive master, if you were so inclined. His tours depart from Mismaloya, about 25 mins south of PV, so you get to see a little bit of the south side of the bay, which offers an excellent ocean view for the entire way. On Cesar's dive was my first sighting of sea turtles and rays.
PV Sea Dive is also a good shop. The owners are 2 ladies from England, and they are both an absolute pleasure to dive with. Very casual and friendly, but thorough where it matters. The cool thing about PV Sea Dive is that their boat leaves from the Marina, so on the way to and back from the dive site they are always on the lookout for whales migrating through, so you get a bonus whale watching tour on your dive trip! Both times I've been with PV Sea Dive we've seen humpback mothers and calves, at one time coming within 10-15 feet of the mother when she surfaced suddenly near us -- AMAZING. If they know whales are there, they'll take you there. Really nice touch to the experience.
Personally, I recommend you dive with them both.
Since you're going in a month, the water will be a bit warmer, the tourist season will have simmered down, and you'll have no trouble booking a dive on the spot. They will be happy to see a diver for sure! They may even have discounted rates for low season. I've heard that it's a whole different ballgame out there when the water is warmer. Regardless, if you love to dive, then there's no reason not to give PV a shot.
Have a great trip!