I took Overlord's full-day tour to American D-Day sites today and thought it was a very good value at 100€. There were 9 of us on the tour. I'd guess the van might hold 20 or so passengers. They do have smaller vans as well.
The company is often recommended, so I thought I'd pass along some elements of my experience that might be useful to others.
If you're trying to arrange a last-minute booking (for the current week), the website provides an email address to use in lieu of the web inquiry form. I emailed twice and never received a response. Using the web form elicited a response within 24 hours. So I'd suggest using both methods if you need a quick answer.
Mentioned in the FAQs but not described elsewhere that I could find is a tour (Tour 2FS) designed for folks not staying in Bayeux who nonetheless want to take a full-day tour. It runs from 9:30 to about 6 and begins at the Bayeux RR station. The standard Tour 2 begins at 8:20 AM and embarks from a location in the heart of town. I assume Tour 2FS doesn't run very frequently (heaven knows, it's not well-promoted on the website), so I think it would work best for folks with considerable schedule flexibility and/or those traveling in groups that would themselves fill several seats in the van.
On Tour 2 I believe I walked a total distance of roughly 4 to 4-1/4 miles, spread out over all the stops (which I didn't count). You could walk a bit more than I did at the American Cemetery; I opted to focus on the museum. You could also walk somewhat less without missing any of the driver's explanations. The walking was not difficult, just sometimes a bit uneven, across grass, etc. There were very few steps.
On my tour the lunch break was at St. Mere Eglise and lasted from 1:15 PM to 2:45 PM. That is also the time during which you can see part or all of the Airborne Museum (entry fee included in tour). By buying a sandwich at a boulangerie rather than taking time for a sit-down meal, I was able to see a bit more than 2/3 of the museum. It is a worthwhile stop, though I thought not quite as professionally put together as the museums in Bayeux and Caen. Those wanting a chance to see all of the displays could take something for lunch with them. There are some park benches near the church where you are dropped off. Eating and drinking in the van is not permitted.
I don't remember how much time we had at the American Cemetery and Visitor Center at Colville-sur-Mer after the guide turned us loose; perhaps 45minutes or so. The museum has a film that runs either 16 or 18 minutes. With just a very quick walk through a couple of rows of the cemetery, I basically had to choose between watching the film and looking at the exhibits. I chose the film, which turned out to be very personality-focused as opposed to descriptive of military activities in the Normany theatre. It's possible that the exhibits have a similar focus, in which case some visitors might opt to spend their time in the cemetery. Entry to the Visitor Center is free.
Tour 2 covers the American sector, and the commentary focused on activities in that area. I think people wanting a broader experience would enjoy also taking the tour that covers the British/Canadian sectors. Overlord offers others tours as well.
The weather in Normandy is rather unpredictable and variable. No matter what the weather forecast, I would expect a mix of sunshine and rain. The coast was much cooler and windier than Bayeux today and had some rain and isolated thunderstorms that were not in the Bayeux forecast and did not affect Bayeux. The van is locked at each stop, so taking along an umbrella and rain jacket is not burdensome, though I'm not sure how well a folding umbrella would stand up to the wind...
I thought there were sufficient toilet breaks, though they were at sightseeing stops, which reduced ones sightseeing time a bit.