First of all, my sympathies to everyone involved in a situation like this. It's horrible to be desperately unhappy but without a "good enough" reason to leave, such as abuse or an affair. Guilt is a very powerful emotion and one of the hardest to deal with. It leaves its aftermath on the partner who instigates the breakup so that they are often just as miserable after the divorce as they were before. That's why a marriage should never be broken up lightly. The guilt will pass, but it passes quicker when dissolving the marriage is really the right thing to do.
I think it's important to distinguish between a marriage where the spark has gone and everthing has become dull and boring, and a marriage where one (or both) partners has lost all respect for the other as a result of problems.
Everyone who has been married has, at some point, asked themselves the question "Is this it?" with a sense of despair. It's not helped by the media bombarding us with images of insanely happy couples with oodles of passion. It's not realistic to expect a marriage to be like this all the time though. Think how we'd all hate it if our partners expected us to be cheerful, glamorous and sexy at all times. Neither can we expect that of them. When a marriage reaches this stage, it can still be saved by really good communication and an effort to keep the spark alive by spending quality time together without the kids. It's not easy, but it's possible.
Sometimes, though, it's not enough. My first marriage ended because I lost all respect for my husband (my second ended because of abuse, but that's a different story). Intellectually, we had a great relationship - we were never stuck for anything to talk about. Sexually the marriage was quite boring, but as great sex wasn't the main bond in our relationship it didn't really matter. We were friends first, lovers second. What really did it for us was his lack of ability to face problems. He was an ostrich, and after years of bailing him out, I just had enough. To the outside world, it would have seemed that Relate or something would have fixed our marriage. We didn't argue, nor did things get bitter when we broke up. I just lost respect for him, and when respect goes, love follows quickly. We didn't have children, so the decision to leave was very obvious to me. I can imagine how guilty I might have felt if I had children at that point. I don't think it would have changed my decision though. I know myself well enough to know that if I had stayed the lack of respect I felt would have eventually spilled into a rather nasty form of resentment, where I would have belittled him and emasculated him to the point where I would have looked in the mirror and despised myself. I also felt that he deserved to have the chance to meet someone else who would truly love him, not someone who just put up with him.
Sometimes, a partner can become the focus of everything that is wrong in our lives. Instead of soul searching and finding the real root of the problem, we project our problems on a person and embody them in that person. Leaving a marriage in these circumstances may provide temporary relief but the same is likely to happen in the next relationship until the root causes are discovered and dealt with. Counselling, alone or together, can really help with this.
Marriages should never be ended lightly. But that said, it is IMO a good thing that we live in times where we no longer have to endure years of unhappiness, which is soul destroying for both partners. Deciding to end a marriage isn't based on a points system - if you score over 30 it's ok to leave - but an honest evaluation of your own feelings.
A lot of posters have raised some good points. Ultimately, you have to ask yourself if you are capable of being the best person you can be within your marriage, as a mother, as a partner, as a friend, as a employee (if you are one), and, most importantly, as an individual. Some people can be happy regardless of whether their marriage is the most fulfilling or not. If there is no acrimony in their marriage, and they have found fulfilment in other areas of their lives, then they settle for what they have. If that doesn't sound like you, however, then it is time to leave. Life is too short to be miserable for most of it. There's no right or wrong choice per se, just a right or wrong choice for you, and making the right one will depend on how well you know yourself.
Good luck. I hope you find a solution that works for you.