Good info and advice already. From someone who has used DLR's MP on 3 trips now, and WDW's FP+ on 3 trips now, a few more comments.
First, I think MP is a no-brainer except in special cases where maybe your group are grandparents and young kids who can't do the headliners. Or you will be arriving late to the parks or leaving early. Any day you plan to spend most of an entire day in the parks, I think they are a no-brainer. If you have the parkhopper option, they are a no-brainer times 2 because they really help with getting onto opposite park rides like RSR and GOTG.
Once your are in the parks, MP feels a lot like FP+ and kind of works like it thru your phone. Some key differences once you are in the parks:
1. Like legacy paper FP, the FP/MP distribution starts with a morning window and moves forward throughout the day - unlike WDW where you can choose your window (in advance of course, unlike DLR) but also day of when you are in the parks.
2. You only choose one MP at a time, but can hold multiple FP/MPs if windows for previous MP have not yet arrived.
3. Other guests who have MP can cancel their MP and thus switch to a different MP ride. What that means for you is sometimes a MP comes up for an earlier time frame even though the moving window has passed. For example, say you are trying to get a MP for Space Mtn. And the current window you see on the DLR app says 7-8PM. But right now it is Noon. Keep an eye on MP because if someone cancels, out of the blue a Noon MP will pop up and you can grab it. This is sort of like how FP+ works at WDW when others cancel. This also applies if all MP/FPs are distributed for the day and none are available. For those using paper FP, they are out of luck for FP that day. But for MP users, MPs can become available at this point if other cancel. So you need to keep refreshing the DLR app like you do at WDW. Along these lines, if I have MPs and leave the parks for the day and know I cannot use them, I cancel them so other MP users can grab them. Sort of like the old legacy FP days when you spread some pixie dust and gave your FP paper passes to a stranger when you knew you could not use them.
4. DLR occasionally has "Instant FPs" that show up in MP. The traditional wait window of 40 minute is not used. You can get a FP for a headliner ride, even though the standby queue is 60 minutes, and if it is instant MP, just grab the MP on walk thru the FP line and pass the standby queue.
5. You can book MPs at the opposite park at any time. The two systems at DL and DCA are linked. So if you are at DL and have an open window for MP, you can get an MP for anything at DL or DCA. And vice versa.
6. When you take a lunch break or afternoon break to the hotel, you can continue to accumulate MPs.
7. Similar to WDW, if your FP is for a ride that goes down in your window, you get the ability from your phone to change it to other rides. At DLR they oftne just give a "MEFP" Multiple Experience FP which you can use on (almost) any ride at any time. Even on rides that do not offer FP. Just show the MEFP to the CMs at the ride and they will let you on. Similar to WDW, the most popular one or two rides are usually excluded from this.
This a great explanation! One really newbie question from someone who hasn't used MP yet: How do you refresh the app to look for different FP's for a specific ride and/or time?
Photopass
And, as others have said, if you want Photopass anyways, that offsets the cost of the MPs. If your group is small it may entirely cover the cost.
Since DLR does not have MagicBands (yet), then you have to claim each photo you want and it will send it to your DLR app for download. You cannot rely on a MB automagically sending your ride photos to your account like WDW does (something I think is
extremely cool BTW). What I do at DLR is after I get off a ride, I take a quick photo of the ride photo that displays at the end of the ride so I can get the photo ID for FP. Then right away, or later that night (or really any time over the next 30-60 days) you can type that ID into the DLR app (or on your DLR account on your computer or tablet) and get your photos.
Hope that helps!