You might want to consider waiting for the stand-alone Rebellion expansion which will cost $40 but will contain all of the content from the previous games. However, if you purchase the Trinity collection now it will save you $10 on the Rebellion download. So, it depends on the price of Trinity and how much you want to play the game. (If you have a huge amount of free time over the Summer then maybe it would make sense to get Trinity now.)
This game is about 3.5 years old now and the online multiplayer (Ironclad Online) player counts aren't real high. However, there are normally a couple 4v4 and 5v5 games everyday on the Diplomacy expansion. (Regular Sins and Entrenchment are pretty dead.) It's a little more active on the weekends. The peak of activity is generally from around 3 pm to 11 pm U.S. Eastern Time. During that time player counts on the Diplomacy server can range from 25 people up to 50. However, half or more than half of those people are just playing against the computer with their friends. Most likely, there's one 5v5 and a 2v2 or 3v3 being played at any one time. As things stand now, you will have to wait to get into games. Hopefully hundreds of new people will come online to play PvP when the Rebellion game (which might be like a Sins 2) is released.
What the other players have said about a learning curve is correct. It's hard to break into this game now because most or almost all of the people in the 5v5 games are experienced pros. There are still some newer and lesser-skilled players around, though. If you get the game you'll probably want to spend your first month playing against the AI until you have a solid grasp of all the mechanics, the races, their strengths and weaknesses, and the tech trees. You will get thumped when you start to play other people because human opponents are very different from algorithym-driven AI, so you have to learn how to play against other people. You have to acquire experience to be able to predict what other people will do, how they will react to things, and very importantly, to be able to assess the strengths, weaknesses, and capabilities of opposing fleets and defenses.
However, it's an awesome game once you get up to speed. Some new guys pick it up faster than others. If you are diligent and serious about getting up-to-speed, it's possible to be able to hold your own with the pros in about two months (one month playing against single player, one month playing against pro players). Also, you can read the Strategy forums and watch replays of games, which allows you to study and break-down every move the pro players make. (So even though you might get thumped in your first 50 games against other people, they can server as treasure troves of information.)
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