I have a 550EX I got for my 300D, mainly because the built-in flash has a very poor reach (low guide number) for my purposes. The built-in flash works OK outdoors for fill-flash shots, as long as you aren't more than about 6-10 feet from the subject. I opted for the more expensive and more powerful 550EX, so that I could easily set FEC on it. The 300D doesn't support FEC on the camera out-of-the-box and none of the flashes will do it themselves, except for the 550EX, although there is a firmware hack floating around that can assign FEC to the 300D 'Set' button, I believe.
You can always use the 550EX with a better body or as part of a multi-master/slave flash setup, later, as well, so I consider it a good investment if you are getting into Canon gear for the long haul.
The 550EX works fine as a bounce flash mounted on the camera, for landscape (horizontal orientation) shots. If you rotate the camera for vertical shots though (BG-ED3 vertical grip highly recommended), you will throw ugly shadows on walls to the right and behind your subjects, because the flash will now be to the left of the lens. This also happens with the built-in flash. You can help this some, by setting the 550EX for vertical bounce, and using a soft/diffuse filter, but the shadows will show, nevertheless.
What you need is something that will hold the 550EX vertically over the lens when you rotate the camera. The only thing I've found so far that works with the vertical grip installed on the camera, and still keeps the flash centered over the lens, is the Stroboframe Pro-T. You can't use the smaller Press-T, because there isn't enough clearance for the camera and grip, especially with something like the off-camera-flash cord or most certainly, the STE2 transmitter.
Some folks have complained that the Canon off-camera cord (OSC2) cable tends to break from all the flexing, and that the flimsy plastic flash mount has broken, dropping the flash. The recommended (pricier, but at least you won't be replacing broken components on a regular basis) is to use the STE2 infrared transmitter. This has the advantage of eliminating the strobe cable, and also lets you place the flash wherever you'd like, for unusual lighting effects.
I know it sounds like a lot, but if you are going on a once-in-a-lifetime cruise, you really don't want to cheap up and miss out on some great shots, because you didn't have the flash equipment you needed.