Efficiency is greatly improved. Stylishly designed, aesthetically pleasing. New Finder windows are better organized.
Quirky, somewhat detrimental changes in some cases. New interface may unsettle some users.
The Desktop: two steps forward, one step back
The first five versions of Mac OS X largely clung to the same formula for the very front end of the interface: while pinstripes have come and gone, the white menu bar and flat dock have always been a warm, familiar sight for most Mac enthusiasts. This is probably why the new mirrored-finish dock has startled and even angered some users: while the design is largely a cosmetic makeover, the sheer overtness of the change may be jolting enough to make some reject it out of hand.
It would be easy to think of the translucent menu bar as a cynical ploy by Apple to draw in Windows switchers, who might be comforted by the glassy look of Windows Vista (or at least, by a visibly different Mac OS). While there may be some truth to this -- no one had argued that the solid white was fundamentally broken -- I found in practice that the bar was more tasteful than its Microsoft half-brother and could actually look good in the right circumstances. The primary flaw with the gossamer skin lies with using solid colors for a desktop background: any particularly bright color, like blue or pink, tends to be garish with the new look while subtler shades can often be acceptable or even attractive. Legibility is simply not an issue, though the menu can be distracting alongside certain windows.
The Dock, however, includes changes that easily cut both ways. Contrary to expectations, the Dock's 3D, reflective look is not especially jarring or harder to use than the 2D design from Tiger and earlier OS releases; the only real issues are the comparatively unprofessional look and the likely performance hit for systems closer to Leopard's minimum requirements. Thankfully, positioning the Dock on the side produces a less demanding, but more conventional 2D version. An unofficial Terminal hack is also known to bring this dock to the bottom view, though I believe Apple should allow users the option of choosing this Dock style with a future update.
The Stacks feature proves to be the true mixed bag of the Dock. The feature is clearly designed for first-time and clutter-prone users who would otherwise lose track of icons on the desktop. For Apple's intended purposes of managing Safari, iChat, and Mail downloads or quickly accessing a handful of documents, the concept of a stack works beautifully: a new file is only ever two clicks away. In real-world circumstances for some users, though, Stacks breaks down. It only works for relatively small file groupings: those of us who tend to keep large numbers of files in Documents or similar folders will be dismayed to learn that many of their files simply won't show up in the stack, forcing a visit to the Finder. It also prevents the easy use of the Dock as an application launcher and tends to produce a large number of similar-looking icons; like some early reviewers, I prefer the choice to revert back to the list view for folders mounted in the Dock.
the Finder: an overdue improvement
More than any one feature of the OS, users will have to contend with the Finder. Much to my relief, this is perhaps the one area Apple spent its greatest development time, and it shows in most every component of the file browser.
The iTunes-like style of the new Finder is a welcome replacement. The brushed-metal appearance of prior versions often clashed with nearly every other window on the desktop. It provides Apple with badly needed room for icons that simply would not have fit with the old design. Although the "search for" list is somewhat redundant, the view can now include all the common folder shortcuts (now known as "places"), mounted drives, and network volumes without expanding the window. It also streamlines one of the most overlooked aspects of previous Finder versions: network sharing. By default, any detectable Macs and PCs automatically appear in the "shared" list, making it easy to see which systems are online and connect to them quickly. Leopard's handling of disconnected volumes is also far more graceful: a sudden drop from the network no longer stalls the Finder until it gives up on reestablishing the link.
Leopard includes Cover Flow as a default Finder view, but it only occasionally proves handy for folders full of photos or videos; I still prefer icon or list view. The former view now has a white border around images for visibility's sake, and each view mode can take advantage of a long-awaited live icon preview, right from the Finder. Anyone who has ever had to wait while Microsoft Office loads just to check one page in a Word document will appreciate Quick Look: the feature really does live up to its name, allowing users to quickly view a document in its entirety, even something complex like multi-paged Excel spreadsheets. It could actually serve as a full-blown replacement for playing movies in full-screen, since it still includes the basic pause, track scrubbing, and volume controls but loads faster than the dedicated Quicktime player.
When it comes to minor elements of the Finder, Apple has paradoxically fixed some issues while breaking perfectly functional ones. Users can finally set not just size and font properties of icons but also their spacing on the grid: this cuts down immensely on the amount of wasted space in large Finder views. Stronger visual cues now also make it more apparent which windows are active or at the front. Even so, Apple has apparently disabled the hierarchical approach to folder views; as Ars Technica's view of the Finder reveals, Mac OS X no longer automatically sets the folder view for subfolders based on your preferences for a top-level folder: unless specified through view options, every new Finder window uses whatever was set for a system-wide view. This regression from Tiger is strange and could prove frustrating to those unaware of the change.
Even with these apparent quirks, it would be extremely difficult to step back to Tiger just based on the Finder alone. While the changes to the desktop are somewhat redundant, the Finder is just that much more functional and pleasant to use. From a certain point of view, the upgrades could rightly be seen as arriving late -- Windows' network view and photo folder views have both been present for years -- but there is no question that Leopard has made strides forward on this basic level, creating a practical, aesthetically pleasing environment to work in.