If you have made the investment in a 5D MkII I would suggest adding the 580EX II to the equation. In all honesty I would look at getting 2 580s which would give you a mobile studio. In terms of compatability, any of the Canon speedlites will work. The 580 delivers the most power which is important because more power =more flexibility. I hear people say the 430 delivers enough power to light anything in any situation. This is true, but you often have to run that 430 at 1/1 power where the 580 would be at 1/2 power, meaning faster recycle and 2x more pops per battery charge. The 580 is also the only Canon Speedlite that has a PC cable connector. Trust me - that is a big deal. I use 2 580s and I can shoot almost any situation without being tied to a studio.
It is a very powerful flash. You can bounce from any light coloured surfaces upto say 50 feet. Learn to use flash exposure control, cause usually it is too powerful on standard settings I find. It also works remotely off camera for a better lighting angle.
Canon usually has rebates at least twice a year- usually one in the spring and one in the fall. The fall one has ended, however, a spring promotion should be coming up soon. Some are better than others. For instance, my first 580exii was a sweet $75 off, but my second purchase was only $25 off. For active rebates for Canon purchases, amazon had usually posted the related products as instant rebates. Check out http://www.the-digital-picture.com/Canon-Lenses/Canon-Lens-Rebates.aspx for a history of them and the Canon website http://www.usa.canon.com/cusa/professional?pageKeyCode=53 for active ones.
Actually the coupon you speak of, from B&H is not a coupon. The price they show is the price AFTER instant rebate! I suggest you visit the Canon website and it will explain how the rebate works. I hope Amazon gets them instock at the $350 again and I will be ordering it. I sold my original 580 and want the newer one for my 1D Mark 3. Esp here in Alaska, the weather sealing and that metal foot will be nice. Thanks
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The answer, of course, is "it depends."
The 50mm 1.4 gives you 4 stops more than the 18-55 kit lens at the long end, which is a lot more light. However, the depth of field at f/1.4 is very shallow. If you're shooting subjects at "across-the-room" distances, depending on what look you want, you may find that shooting the 50 wide open doesn't give you enough depth of field. It also isn't very sharp at f/1.4.
Putting the 580 on your hot shoe isn't going to magically make the "deer in the headlights" look go away. You'll need to soften the flash by bouncing it off a wall or ceiling, or get it off the camera and onto a lightstand so you can use an umbrella or softbox. If you really want to fall down the rabbit hole I suggest you check out the Lighting 101 tutorial on the Strobist blog (http://strobist.blogspot.com/2006/03/lighting-101.html).
All that said...the 580 is a very big, very powerful speedlight. I recommend you check out the 430EX II, which will give you much of the power and many of the features of the 580 in a smaller package, at 2/3rds the price. That's what I use on my XTi.
Yes, the 580EX II will work with the Rebel T1i, but, is probably overkill.
Look at the Canon Speedlite 430EX II Flash instead.
The 430EX II has a guide number of 43 meters/141 feet @ ISO 100, compare that to Nikons top of the line SB900 flash which has a guide number of 40 meters / 131 feet @ ISO 100. Unless there is a specific reason you need the 58 meter guide number of the 580EX, like night football, just go with the 430EX II.