ME3 has no problem standing shoulder-to-shoulder with its predecessors, but offers an unsatisfying end to the saga.
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After finishing Mass Effect 2, the long wait for Mass Effect 3 was all but unbearable. The culmination of this epic saga is, for players of the previous games, the culmination of possibly hundreds of hours of gameplay and a story that keeps the player glued to his seat. Unfortunately, this pushes expectations so high that massive effort must be made to satisfy them - and it seems that Mass Effect's writers were not really up to that challenge.
Mass Effect 3 picks up several years after the end of Mass Effect 2, with the Reapers finally invading the galaxy - putting the brunt of their assault against the most dangerous target (as they perceive it): Earth. The entire game revolves around gathering allies and assets in order to push the Reapers off Earth, and so continues the theme of galactic cooperation that ran through both of the previous games (whether or not Shepard's actions actually contributed towards that end).
Decisions made in previous games may have a huge impact on the outcome of Mass Effect 3, as they can influence the size and strength of the assets that can be collected to that end. By choosing, for instance, to save the Rachni Queen in the first game, you might be able to recruit the help of this race to your side in Mass Effect 3. Kill the Queen, and things can look very different in the end. This is a very nice touch, and would've been a great feature to have in other long-running stories such as the Assassin's Creed series.
Strangely, however, one of the more important gameplay elements of previous games - namely the morality system - has been made largely moot. Yes, moral choices do make a difference, but the distribution of Paragon/Renegade points is irrelevant. So long as Shepard continues to gather points in either category, options to make moral choices of either kind become unlocked. You can play a Paragon all the way through, and then at a key moment pass an intimidation check just as easily as if you'd played Renegade the whole way through. Furthermore, the ending (which is discussed in more detail below) is completely unaffected by whether your Shepard is fully Paragon, fully Renegade, or neither.
This is just one indication that Mass Effect 3 has taken additional steps towards First-Person Shooter territory - which comes at the expense of its role-playing aspects. A disproportionally large amount of the game is spent playing combat missions, many of which are nearly devoid of story altogether. It's true that previous games has many such missions as well (particularly the first Mass Effect), but in this case there is not enough content outside combat to justify them. My assumption is that with multi-player mode being added to the game, BioWare focused a lot more on enhancing the combat portion rather than spending their time on the story/RPG parts, ultimately to the game's detriment.
Though it's hard to compare the amount of dialogue in all three games, it certainly feels like there is much more combat to be had, fewer chances to use charm/intimidation, and far less exploration. Though some players will see this as an advantage, it makes ME3's galaxy seem smaller and less interesting than the sprawling campaign of ME2. Even the planetary exploration mechanisms, which admittedly were poorly executed in previous games, have been replaced (yet again) by a completely new system that is utterly uninteresting and seems much more action-oriented (but is really just repetitive and pointless).
Of course, the reduction in dialogue can be justified in a narrative sense due to the war going on in the background and the fact that little exposition is needed by this point in the story, but it does make the game a little less interesting and more repetitive. For players wishing the game was a little longer and larger, two additional DLCs ("Leviathan" and "Omega") will add at least a dozen hours of extra content which is highly recommended.
Several new and "revised" characters appear in ME3, particularly some new and interesting party members. The "From Ashes" DLC is also highly recommended as it adds a very interesting and surprising squad-member that I personally enjoyed bringing along on missions. Many characters from previous games will reappear (assuming you haven't gotten them killed yet), but most veteran squad-members are there for just one mission or so - just to wrap up their storylines before the trilogy ends.
As pointed out by many in the past, the game's ending is pretty much the only serious flaw. For starters, the "Extended Cut" DLC is pretty much required. BioWare had set the requirements to reach the "best" ending so high that you either had to play some multi-player matches (a bad call for which BioWare was derided) to get the necessary score for that ending, or had to take an extremely specific path through the entire trilogy (usually means playing the whole thing from the start with a walkthrough in hand). Fortunately, the "Extended Cut" reduces the requirements for the "best" ending, so as far as I'm concerned this DLC is absolutely required.
Neither of the two endings offered by default are truly "satisfying", lets just say that they are bittersweet at best and totally facepalming at worst, depending on your expectations and how well you do during this final sequel. "Extended Cut" adds a third ending, but it too is rather bittersweet and thus not much more satisfying than the others. To make matters worse, the downsides of each of the three outcomes (based on a single choice made at the very end of the game) are actually discussed just before that choice happens, further punctuating the fact that none of the options presented are really sensical.
I guess it would be hard to write a satisfying ending to such a massive story, but perhaps BioWare should've put more resources and time into finding the solution to that problem rather than rushing the game out. I'm not privy to their considerations, but they really are to blame for the ire was brought upon them due to this decision. Again, even with the DLC released to "rectify" the problem, BioWare have not managed to provide suitable closure - and that is easily the worst part of ME3. After all, this game is the ending of a saga, there was really nothing more important than providing closure here, and they messed it up.
So to summarize, ME3 is certainly worth the time it will take you to play, with the same high-quality production value, familiar and trusty combat mechanisms, and plenty of story to go through before the trilogy ends. However, it can be extremely helpful to lower your expectations before the ending - as much as that hurts given all the time you've spent playing the previous games to this point - otherwise it can easily disappoint. And that disappointment may end up hurting your enjoyment of not only this but both previous games in the series.