On first inspection, Illustrator CS5's interface is much the same as in CS4 and features the same spring-loaded palettes that can be viewed either as icons or in their expanded form. Although there are of course slight differences in the overall look, there is nothing here that will hinder your usual workflow.
It is the smaller additions - those not billed as the 'big-sells' by Adobe - that will ultimately be of real benefit to a workflow. The Shape Builder tool is one of these. Anyone who uses Illustrator regularly will know how fundamental the Pathfinder tool is; well, the Shape Builder tool offers an intuitive alternative for shape building, enabling the user to add or subtract two objects by selecting them both and drawing a line between them.
I'm always sceptical when software programs try to emulate the look and feel of natural media - if you want it to look like paint, just use paint. The Bristle Brush tool does little to change my mind on this matter, and in trying to achieve the look and feel of real-world brush strokes all it manages to do is create some rather unconvincing 'wavy' effects. Having said that, some interesting effects can be achieved by playing with the variables in the new Brush Definition menu, and I can see myself using this new tool in the future - but perhaps not for the exact purpose it was intended.
The new Variable-Width Strokes tool has the potential to shave valuable time from a job. By selecting individual points, you can manipulate a path's width at any given point - a godsend if you're working with lines but also want to suggest form.
CS5's new Drawing in Perspective function is the biggie that Adobe is selling the latest version of Illustrator on. It's certainly a welcome addition, and enables you to quickly draw in 1-, 2- and 3-point linear perspectives. You can now create and edit your own perspective grid, deciding where you would like the vanishing point to feature and how severe you would like the horizontal and vertical planes of perspective to appear. Once you've set your grid up, you can then either draw directly onto the planes of perspective or import some of your own existing 'flat' vectors and add them to the grid, quickly and easily giving them a sense of depth and perspective. This tool is easy to get to grips with but will probably take a little longer to really master, and I don't see it convincing creative professionals who work predominantly drafting and creating technical drawings to switch from their current software program of choice. I do, however, see it being very useful for the commercial illustrator looking for a simple way to add a sense of depth to their work, and I look forward to playing with this further myself and seeing how it's used creatively within other people's work.
The Draw Inside function enables you to quickly mask off areas and place images within them, or draw or paint directly within them, negating the need for clipping masks. After exploring this for a while, I don't see myself working with images in this way as I would alternatively opt for one of the other programs in the Creative Suite when dealing with images. I do, however, see it being useful for creating cropped areas of interest within a piece without using clipping masks or using the Pathfinder palette.
The Draw Behind function is a nice little addition to CS5 that enables you to draw directly behind any object within your artwork without having to tediously navigate to the right location in the Layers palette. It's not a game changer but it's certainly a time saver.
Overall, I would say that this is a welcome new version of what is a great drawing program, and it certainly feels like Adobe has tried to move things on a bit. As is often the case, though, I think it will be the smaller, less 'whizz-bang' additions that will ultimately prove the most useful and actually have the potential to speed up your workflow.