Just wanted to post a few thoughts about my six day trip to Dominica, in case they are helpful to any others.
After reading the forums, I was very nervous to rent a car and drive around. But there were plenty of tourists with cars, and they all seemed to do fine. And it definitely would have made day trips easier. We stayed in three hotels while were there - Titiwi Inn in Roseau (wanted to save some $$), Cocoa Cottages in the Roseau Valley, and then Calibishie Cove. Cocoa Cottages was WAY closer to Roseau than we realized, and the bus passes right by the hotel. So if you don't want to stay in Roseau but want access to the amenities in the city, it's no problem to stay in the valley.
The only waterfall we had time for was Trafalgar Falls - I would have rather seen Sari Sari falls or one of the others, but we just didn't have time. If you do go to Trafalgar, go around 5pm - the cruise ships are gone, and it will just be a few locals. We didn't make it up to the actual falls - couldn't figure out how to get over all the boulders! But it was nice to relax in the warm pools. We went to Tia's for a little; didn't seem worth it to me. There are plenty of natural warm springs to bathe in in Dominica; why spend your time in a man-made pool??
If you're up for the long trek, definitely do the Boiling Lake. For the majority of the trail you really don't need a guide except that at the very end the trail is hard to follow. So if you want to make it, take a guide! You can actually scramble down to the shore of the boiling lake, which was a cool spot. There are also some perfect perfect hot pools along the way. It's really some gorgeous scenery.
I didn't think that staying in three different spots was too much - I liked the change in scenery, and of course we never got bored. Although, at every spot there were things I wish that we had had time to do, and that we never got to see.
I think that transportation is the toughest piece to plan for a Dominica trip. Here's what we did, please feel free to message me for more detailed info!
Airport to Titiwi Inn: We booked a cab through the hotel. I'm not sure if buses actually go to/from the airport. While staying at Titiwi, we walked to restaurants and into town (not through the best neighborhood, but it was fine). There are plenty of buses into the city, and we also took the bus to Scott's Head - very easy. We got suckered into taking a "tour" to Trafalgar Falls and Tia's, which I thought was overpriced, but I guess it's the going rate (something like $35/person plus entrance fees). Just take the bus to Trafalgar Falls.
We took the bus from Roseau to Cocoa Cottages. Next stop was Cocoa Cottages to Calibishie. We ended up getting a ride from Cocoa back to Roseau, then hopped on a bus to Portsmouth, quick transfer to a bus to Calibishie, which dropped off at the road to Calibishie Cove. Everyone was very willing to point us in the direction to the right buses.
While in Calibishie - you can walk into town, although it's somewhat far (40ish minutes). We walked, but there were some buses going by that we could have jumped on. Someone from the hotel drove us out to a gorgeous beach (I think it was Batibou), and then we walked back (took a long time, maybe an hour and a half). We booked a taxi from Cocoa Cottages to the airport.
Final tip - We got to the tiny airport exactly 1 hour before our flight, and they told us we were late. Then our flight left 20 minutes early. So get to the airport in advance!