c&c
When you discuss the merits of C&C vs. 3.5e or 1e etc., I think it is important to remember that most people on these boards by definition are huge role-playing game fans and have at least dabbled in numerous versions of d&d, c&c, and other similar systems. I love playing C&C but then again I also really enjoyed 1e d&d and 3.5e d&d. The fact that I enjoy at least some aspects of each of these permutations of our favorite RPG does not mean that the versions are totally parallel. Given that many individuals have preferences to style, content, speed of play etc., I think it is desirable that many will gravitate towards one form or the other. The question is what type of game best suits your personal preferences as a player or a dm. Factors to consider when you analyze your preferred style of play (and which game you ought to consider) include:
1) SPEED OF PLAY. How do you enjoy spending the bulk of your time when you are actually in session? On a scale where basic d&d is the simplest and fastest version of d&d and 3.5e (particularly at high levels) is by far the slowest, C&C is in the middle. Having played all of the versions extensively, I'd say C&C is a bit faster than 1e or 2e because there are no tables to look up, there is a unified saving throw system and a much easier to use AC system (ACs go 'up' like in 3.5e). Complexity tends to be the antithesis of speed so see item #2 below.
2) WARGAMING VS. ROLEPLAYING. If you want more of a 'wargamey optimizing' feel, with long moments spent savoring combat options and the intricacies of various types of movement in combat, setting up potential AoOs for your comrades, and carefully measuring spell radii, then 3.5e is better for you. The extra combat complexity of 3.5e can easily give a greater 'war game' feel. C&C has many more combat/spell options than basic d&d, and a bit more than 1e, but its speed of play vis a vis 3.5e comes from having somewhat fewer combat options/calculations/richness in AoO scenarios.
3) MODULARITY. If you are someone who wants a ready-made core system, then 3.5e is fine. If you love to house rule, then C&C is probably significantly superior. For example, I like some aspects of the feat system in 3.5e. I will port over a much scaled down version of the feat system to C&C and I may create a slightly more elaborate grappling system than C&C currently has. This can be accomplished with zero effort. C&C is incredibly modular and very easy to add to. In my experience, 3.5e is very complex and very interrelated in its rules. You really need to think carefully about these interrelationships in 3.5e before you add house rules.
4) DIMENSIONALITY. Early versions of D&D, from basic to 2e have had a 'dump stat' phenomenon. Some believe that 3.5e went partways towards fixing this by creating an elaborate skill system that is partly tied to attributes. C&C designed a far simpler system that uses all six attributes as the base for its attribute checks and saving throws. For this reason, a player in C&C should be very wary of 'writing off' wisdom or charisma because he is a fighter. In other words, C&C players should be wary of becoming completely one dimensional whereas the proliferation of specialized prestige classes, path dependent feat chains etc. seems to argue the opposite for 3.5e.
5) DM PREPARATION. DMs can spend an enormous amount of time preparing and converting adventures between sessions. My biggest knock on 3.5e is that I find that when I am the DM it takes an incredible investment of time to create the stat blocks of basic creatures. C&C is similar to Basic, 1e and 2e in that the time spend on creating stat blocks is very modest. In addition, C&C is more similar to each edition of D&D than they are to each other, for this reason, it is much easier to translate a 1e or 3.5e module into C&C terms than it is to translate a 1e module into 3.5e terms. In a similar vein, C&C is much easier to 'wing it' for a dm rather than a player.
6) FLAVOR. Because there is less time spent looking up rules in C&C as opposed to 3.5e, it is easier to stay in character (assuming that the player is any good at it to begin with).
7) CHARACTER CUSTOMIZATION. C&C is much more customizable than Basic D&D or 1e -- there are more character classes, feats are incredibly easy to port into the game, etc. On the other hand, 3.5e has an almost mind-boggling area of classes, races, prestige classes, feats, splat books, etc. If you really want that half-tiefling shadowdancer/thief-acrobat than 3.5e just seems to have more resources. Again, given C&C's modularity it would be easy to port into the game a few of your favorite character races (say goliaths) or classes (say sorceror or swashbuckler) or feats (say your 20 favorite), but it would require a great deal of effort to recreate the vast richness of customization possibilities already available in 3.5e. The cost of the advantage for 3.5e in character customization is that it slows down the game and the richness in options can interact in surprising ways and 'break' the game if the dm is not extremely vigilant -- e.g. someone showed me a high level halfling thief/master thrower/xxx who might be able to kill Thor in a couple of rounds with halfling skiprocks. I cant imagine how youd create that character in C&C, even with some power enhancing house rules.
8) ZAGYG IN C&C. This may not mean much for younger players, but Zagyg (i.e. Gygax's original Castle) will be published under C&C rules. many older players have been waiting 20 years for this adventure.
In summary, C&C is a middle ground between editions, that is much better constructed than early D&D editions. It is much richer than D&D Basic in combat options and spells, it is much less clunky than 1e (which had complex to hit and save tables and really odd weapon speed and weapon type vs. armor concepts), and it is far faster to run or prepare an adventure than in 3.5e. It gives up a bit of the 'wargaming optionality' and some of the incredible character customization potential of 3.5e splatbooks to accomplish its far faster pace. Your choice of whether you purchase C&C should depend on the extent to which you value speed and modularity vs. wargaming optionality.