Canon remains as prolific as ever in terms of sheer number of products brought out. Too many products, even, since we, the testers, consumers, and salespeople find it hard to follow what's going on and generations of machines pile up on the overloaded shelves. This year, taking all their computer peripherals together, they have launched one new product every four days! A more limited range would simplify the position slightly.
The i250 is economically priced but is the most limited of the Canon printers in terms of functionality. It cannot print without margins and it doesn't even have a paper collection tray. If that doesn't bother you, you can use it without a problem. Just remember that, after all, this is a printer dedicated to small volume production, less than 40 pages a month. If you intend to do more, opt for printers that use separate cartridges.
This printer is the same as the i250 but a little faster, with a paper tray and printing without a margin. It was priced slightly too high at launch but has been intelligently repositioned. If you forget additional functions (card readers, sensors, etc.), its good value for money in terms of speed, purchase price, and usage.
The i450 is a lightweight version of the i850. It's slower, more economically priced, very good in photo mode and slighter more expensive in terms of ink. So it's a very good choice for people who want a competitively priced printer to print small amounts (less than 40 pages a month) of text and a few photos. It is gradually disappearing from the ranges, however, to the benefit of the i455.