Train and bus will get you pretty close to most good crags. You may have to hitch or taxi the final leg into some places. I didn't have much trouble getting around this way. Watch for public transportation being closed certain days in smaller towns though.
Rent a car! I rented a great car at an astonishingly low rate. I can't remember the company but it ended up being something like 6 euros a day I think. I think we ended up paying more for gas than the actual rental. Also, beware of all the toll roads! Something to consider if you're on a budget. If you rent a private car you pay tolls left and right. If you're on public transit you don't have to.
So it sounds like renting is my best option. Lleida, margalef, and the catalonia region are the places that seem most appealing to me. Anywhere in particular within those places I should be looking out for?
The first 2 options are amazing and highly recommended. The third was really touristy and raining the whole time, so we did not even get to climb. I am sure I would have had a much higher opinion had we actually climbed!
I'll share my thoughts after spending three months there with a rental half the time:
1) With a car: You'll have way more options. Namely flexibility when changes occur in weather, psyche, etc. Not to mention convenience when you want to go to the bar/store/sight-seeing. Some places like Margalef are very difficult without a car, or at the very least limiting. If you rent for a longer period of time (3-4 weeks) it gets very cheap... we were paying around 8 euros a day. Some areas are a bit off the beaten track--Oliana, Tres Ponts, Collegats--and require a car.
2) Without a car: as mentioned, you can get to many places with public transport. I'll add Terradets to the list as it's within walking distance of a train stop, and i thought it was the best tufas we experienced. You can get most of the way to Rodellar via train/bus (taxi will take you the last few hours), and once you're there you don't need a car. Same goes for Siurana. Chances are once you get to a climbing spot you'll make friends who have cars and are looking to split gas to the next destination, which is what happened to us.
I've had two good climbing trips to El Chorro without a car. There's frequent train services from Malaga airport to Alora, and less frequent ones all the way to El Chorro itself.
If you stay at the Olive Branch (which I would recommend), Gary and Mel will do a shuttle service to El Chorro (very cheap, maybe free?) train station, or Alora (5 Euros per person each way). From the Olive Branch, the crags at Las Encantadas are about 5 minutes walk, and it's only half an hour to various other sectors. A bit more of a trek to Makinodromo, El Polvorin or Los Cotos (60-90 minutes if you know where you're going).
Use the cheapest one. This is not the third world, and virtually all rental car companies you will find at the airport will be reputable. Also, get a very small car, unless you have many people/luggage. From experience, more than once, the rental counter tried to get us to take some large behemoth. Those are a pain in the ass in Europe.
I rented a diesel, which was AWESOME cause I got a VW Golf TDI with 40-50 mpg. The thing was small but has more than enough room for 2-3 people with climbing and camping gear.
I can't imagine not having a car for Catalunya, but I have a hard time staying in one place.
If you are going for a while try and check out Sadernes! It is an amazing place that americans never hear about because there aren't many climbs above mid 5.13, but a shitload of super long 5.11-5.12s! If you do go to Sadernes take a day off to go to the Dali Theater in Figueres, about an hour away. www.salvador-dali.org/museus/figueres/en_index.html
When I went to Spain I was only able to go for 2, too short, weeks. I hit up Margalef (too hot in September), Camarasa, Sadernes and Rodellar. I LOVED ALL OF THE PLACES I WENT, Spanish Limestone is amazing.
Rodellar was the most beautiful, but also the greasiest place we went, Margalef is really neat because the rock is so bizzare (limestone coated cobles), Camarasa is great for 5.11 vert climbing, and Sadernes is a really beautiful canyon packed full of very high quality rock with tufas, pockets, and amazing routes.
The grades seemed to vary a bit at each place, Rodellar was the stiffest (similar to most US limestone crags like AF, Sitting Bull Falls), Margalef was the softest grades I've ever cimbed (at least on the newer walls) and Sadernes was just a touch softer than most US crags (most similar to Red Rocks).
I wish I could go back, but I can't imagine doing so without a vehicle, Spain is amazing, not just the climbing but also the food, history, art, beaches, and people.