I'm hesitant to post too much about Grand Lake because one of the best things about it is that it is so, so, so quiet. But, because these forums turned us on to Grand Lake, I wanted to post a quick review.
Cute, but very quiet, main street. Very walkable. A few nice shops and restaurants. Locals sent us to Sagebrush, which was great! There is a great park in the middle of town. Our favorite spot was the Grand Lake beach and snack bar for ice cream. I would say this is a great, low key family destination. But a younger, hipper group would much prefer Breck or other more hopping places.
Good access to RMNP, but if you want to focus on the east side of the park (which is the most popular side), I do think Grand Lake may be a bit too far for most. This is why Estes is so much more popular. We day tripped to the east side of RMNP for one day, taking a long drive across Trail Ridge Road, which was just enough for us. When we got to Bear Lake Road, we were told it was a 2 hour wait for the shuttle. So, we turned around and enjoyed less popular spots of the park, including Hidden Valley area. Nice visitor center for kids there. On two other days, we hiked the west side. Adams Falls (very short, easy hike) and Cascade Falls (more challenging, long 7 mile hike) were beautiful hikes. Our 9, 5, and 2 year old (in a back pack) made both hikes just fine, but they are pretty outdoorsy kiddos. Adams Falls is a can't miss easy hike for almost anyone. Cascade Falls is a good 5 hours for a young family. Also, kids completed the Jr. Ranger program, which they loved!
We loved our cabin. And I'm not sure there is a more scenic place in Colorado. One note of caution: we did lots of lodging research before our trip. The lodging in Grand Lake is pretty limited, and pricier than I expected. Almost comparable to Estes. A lot of the cabins in the area that are run by local companies seemed dated in the pics. We don't mind dated if the price is right. But we considered them to be quite expensive for the quality. We found much better deals with private cabins/homes. As we drove through the area, we saw lots of the locally run cabins that we had seen online, and it confirmed we made the right choice in finding our own private cabin. However, a lot of the hotels/lodges looked very nice online and also looked great in person. Western Riveria, Grand Lake Lodge, look great! But those were booked when we looked. In sum, the quality of lodging in Estes is better for a comparable price, so if you choose Grand Lake do your research to make sure you are getting something you will be happy with. It takes just a bit of extra work.