They certainly are classics, but they may require some patience for the modern gamer (where here "modern" is defined as "post-1990" -- I love GOG!). For the best gameplay experience, I recommend applying fan-made graphics patches. Here's a link: http://exodus.voyd.net/download.html

The Ultima 2 patch upgrades the graphics from CGA to glorious EGA and also includes fixes for some in-game maps by applying files from the Apple ][ version. These maps were originally corrupted by Origin when they released the Ultima Collection CD, and it's never been officially addressed since then; the Ultima 2 patch corrects the issue. GOG, if you're reading, I even recommend including the map fixes in the game's installer; this is a major issue with the game.

The patches for Ultimas 3 and 4 both upgrade the in-game graphics to VGA and add MIDI music from the Commodore 64 versions. (The Ultima 4 patch is the first link under "Related Downloads".)

The patch for Ultima 5 just adds music.

I highly recommend these patches to anyone thinking of playing these games, as they add considerably to the experience.

With that out of the way:

Ultima 7 is probably the best one to jump into; the interface is far more intuitive than any of the previous games, and the gameplay is just fantastic. Save often, as the game can unfortunately freeze from time to time. The graphics are also considerably advanced over the previous Ultimas, this being the first that doesn't use tile-based movement. (If you are very technically inclined, see if you can get MUNT working so you can hear the MT-32 soundtrack. This post on QuestStudios will get you started.)

If Ultima 7 and Serpent Isle make you curious about the older games, then Ultimas 4-6 should be your next stop. These three games form the moral center of the series: Ultima 4 introduces the system of virtues and the Avatar; Ultima 5 shows what can happen when the virtues become corrupted; and Ultima 6 is all about unintended consequences. Your character file can be imported from Ultima 4 into Ultima 5 and then again into Ultima 6, providing some nice continuity.

The gameplay in Ultimas 1-6 can be intimidating if you're accustomed to mouse-driven interfaces with simple, straightforward actions. Instead, these games are keyboard-driven, to a ludicrous degree: nearly every letter on the keyboard has a distinct purpose! These games were created in a time when it was expected that people read the game manuals, and you'll find a lot of great flavor in the printed material (available as PDFs from GOG or replacementdocs). Have a notepad handy (or, y'know, notepad.exe works too), and write down everything that seems important. The games will not keep track of these things for you. Particularly in the earlier games, your first goal should be simply to stay alive: you need money for food, weapons, and armor; and money comes from killing monsters or thievery. Only once you've worked out how to not die should you even try to advance the plot.

These are all great games if you've got the dedication to play them, and I'm ecstatic that GOG provides them in such a great format. I haven't even touched on the Wing Commanders, which are all fantastic. I'd already purchased every game on this list; now maybe I can convince some of my friends to take the plunge!