I was actually just logging on to this forum to start a thread about Spain's UA; guess you beat me to it.
I usually play Small maps on Quick speed. In case no one is familiar with the values for those settings: 6 faith makes the first pantheon, 134 for the first GP, etc.
In my experience, Spain's UA can be massively gamebreaking if you are lucky enough to be close to Natural Wonders. I've noticed that "Disable Start Bias" usually nets you no wonders, but without Start Bias disabled, then you have a shot at getting a couple. I played two games as Spain: the first one Emperor and the second on Immortal. I essentially cruised to a victory in both.
Generally, strategy consists of building 2 or 3 scouts and sending them in every direction. If you don't find any nearby then congratulations, your UU is worthless. If you do find something, then you get 500 gold, which is enough for you not only to buy a settler, but also to buy the natural wonder even if it's 3 tiles away (at least, on Quick). In my first game I found Mt. Fuji first, so I had my second city within 10 or so turns, and then a pantheon on the next (since Mt. Fuji nets 6 faith per turn with the UU). The other wonders I didn't find first, but I did end up with most of them under my control, after some warmongering. Cerro de Potosi nets you 20 gold, which makes it very valuable.
My second game was true madness: within 10 turns I had found both Sri Pada (2 food, 4 faith base) and Mt. Fuji (2 gold, 3 culture, 3 faith base) and had three cities: my capital and two cities working the wonders. With One With Nature -- which is also doubled -- I ended up getting THIRTY faith per turn. I had a fully enhanced religion on turn 25.
The biggest caveat is that you will doubtlessly stretch your empire thin, since you will expand very quickly and sometimes rather distantly (Sri Pada and Mt. Fuji were 16 and 10 tiles away from my capital, respectively). Not only will this anger your rivals, but you won't have the military to protect yourself. However, if you select "Holy Warriors" (purchase pre-industrial land units with faith) and "Defender of the Faith" (+20% combat strength near friendly city following this religion), you should be able to fend off your attackers. On this game that I played, where I expanded very quickly to grab these wonders, I ended up angering both Elizabeth and Attila the Hun. I didn't have a hard time fending them off.
I also managed to sneak in three conquistadors on the other continent. One was for copper, one was for the Barringer Crater (which is not very useful late game, although 6 science one tile is decent in the very early game; it is comparable to an academy -- it would be a Babylon-like bonus), and the third was smack dab in the middle of the continent for the Grand Mesa (also not that useful). Pacal was mad as all hell, but did nothing, even though two of those cities were right on his borders and I had no military there. Byzantium was very cool with it. I guess Theodora doesn't get mad? America was mad as all hell though. We waged war for the rest of the game, and although he captured two of my city states, he never captured any of my cities, even though the city working the Grand Mesa was nearly surrounded by American territory.
Conquistadors are pretty solid. I don't use them for military except for clearing out barbarians on any continents empty of civs (which has happened to me sometimes). Conquistadors are pretty useful on Large Islands, and pretty "meh" on Pangaea.
My biggest problem with Spain is their second UA, the Tercio. I like that it's more powerful and has a bonus against mounted units, but it's increased production price is a big drawback. It's not often that I find myself staying at Gunpowder for too long, so sometimes I only produce one or two Tercios before I upgrade them to Rifleman. I would rather have Tercios that cost the same as Musketmen BUT are a little weaker, say 24 combat strength instead of 26 and 30% against mounted units instead of 50%. Anyway, that's my opinion about Spain.
------TL;DR------
UA: Fantastic if you scout early/are lucky. Once had 30 faith per turn from Mt. Fuji, Sri Pada and One With Nature in the first 10-ish turns. Once fully enhanced a religion on turn 25. Otherwise, bupkis.
Conquistador: Oddball, but useful if playing on a map that offers the opportunity for "New World" destruction of indigenous culture. (I mean, colonizing).
Tercio: Strong, but expensive. In general, I find that tactics give this unit a short lifespan and limited use.
. Filled up liberty tree fairly quick with that and also got religion in record speeds 
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