We've come to the Fells a few times now since moving to the area last fall and have really enjoyed our visits! If you live in the Medford-Somerville area and want to get away from the city environment for a bit, this is definitely a good spot to go. In addition to some great views of Boston (it can be busy on nice days, but climbing up to Wright Tower is worth it if you want a gorgeous view of the Boston skyline!) and a woodsy environment (you forget how close you are to 93), there are lots of different trails, at a variety of levels (but nothing too tough), for walking, jogging, running, hiking, biking, or dog-walking.
I'm just getting into hiking, and the Fells is definitely a great place to start--we've done a piece of the Skyline Trail to-date, and it's not too tough in old sneakers (although hiking boots would be better). We're hoping to do the whole Skyline Trail at some point--I'm guessing that's going to require at least a half-day commitment. On the parts of the trail we've been on so far, the changes in elevation are quite manageable and the rocky areas aren't too bad--there were a couple of moments where I wished my rather short legs were a couple of inches longer, but I got through them without too much travail. The markings for the paths are so-so... if it's your first visit and you're not good with directions, you might want to print out a map beforehand. We've run across a couple of very lost people on our visits, and the area is big, so that can be a problem (although the paths do cross and loop around one another fairly frequently). The Skyline Trail hasn't been too busy, the days we've visited, although if you go into more popular areas like the Tower and/or stay on easier-to-navigate trails, you'll run into more people (and a bit more litter).
We usually park in or around a small parking area by the Bellevue Pond, which starts you off only a short distance away from the Tower and Panther Cave (less exciting than it sounds). It can be a bit tricky to find a place to park on busier days, although this part of the reservation seems like it's much less busy than the Sheepfold area. Of course, now that I've said that, it's going to be impossible to find parking next time we go...
As others have pointed out, there are a lot of dogs in the park, most of them not on leashes. I'm more of a cat person, so this makes me a little nervous, but generally-speaking the dogs we've run across on our visits have been quite well-behaved. They've generally been more interested in sticking near their owners and checking out all the smells along the trails than in us. Combined with the presence of lots of people, though, this does mean that you're not going to see quite as much wildlife as you would at a less popular park (the area isn't really ideal for serious birders). That being said, we were happy to see a few songbirds (mostly chickadees) and a vibrantly-feathered Mallard duck on our last visit! read more