I don't know where you found it, so I can't say for sure if it's safe or whatever. I haven't seen any free and legal versions of PS, though I also haven't looked.
I can say that I used CS3 for a very long time, and it's pretty good for most things photography related. The biggest issues I can think of between it and the newer PS versions, as it relates to general photography is, it doesn't have all of the fancy automatic subject selection, content-aware replacement, or the better enlarging algorithms of the newest versions. That being said, you can pretty much do anything you need to with it, it'll just take longer and be a good bit more difficult. And I wouldn't be surprised if several of the plugins you find won't work for it, since it's such an old version, it's likely not supported.
Now, I never use Lightroom, even though I have the Master Suite. It's very user friendly, but it's no where near as powerful as Photoshop, and I have a ton more experience using PS. Plus, I have my own file management system, so Lightroom doesn't really help me there. I know a lot of good photographers who prefer Lightroom though. But even then, they seem to use both, because there are certain things you just can't do in Lightroom. I think they just prefer Lightroom for it's ease of use and only go into PS when they need something that only PS can provide. But they almost always start off in Lightroom.