One thing you need to understand about Sigma, they like to call everything macro. If you go look at their lens line-up, I think a lot of them have the 'MACRO' designation. But if you look closely, they are not really macro lenses. Rather, they can focus quite closely...
24/1.8 is a fast wide prime if you use it in FF camera, it will be very wide. On a DX, it gives you FOV of about 35mm, which is a good focal length to work with. Some people calls this a wide normal lens. At 35mm, distortion for people shots can still be managed well. And F1.8 really helps. I would love to have the 24/1.4G but the price range is a little out of my justification reach at the moment. One thing I like about this lens is that since it is slightly wider than a 35/1.8 or 30/1.4, if gives you a little of that wide feeling that make things look a little more 'dynamic', while at the same time, not wide enough to give too much problems in distortion. But the sigma 24/1.8 do allow you to get in very close and shoot some interesting angles. Here is one my people shots with this lens...
One thing though, due to the recent Sigma price adjustments, this lens is no longer that cheap. It costs over $700. And with all Sigma lenses, they really shine if the focus is spot on, if not, you need to send it in to Sigma Service center to get it calibrated since your D90 does not come with AF fine tuning.
Also, for me, F2.8 primes do not make sense to me personally. This is because I already have a 17-50/2.8 which is quite sharp wide open. So if I get anything F2.8, it is just repetitive for me. And I need apertures faster than F2.8 for ambient light shooting.
As for what is really useful for you, you need to understand your shooting style, and from that determine your needs in terms of focal length. Not everyone shoots the same. You have to find a mix that suits you. All I can do here is to give you suggestions and some ideas. You have to see if they fit your shooting styles/needs.