After using VM for many many years, I switched to mu4e a couple of years back. I think VM was/is a truely remarkable bit of software and one which was way in front of many other solutions for a long time. I think the time and effort Uday has put into keeping it alive is highly commendable and I would like to send my thanks for his sustained and considerable effort and dedication. Thank you Uday.
Just in case it helps, here are my reasons for switch from VM to mu4e and my experiences with it, both good and bad. My hope is that others might find it useful.
My main reason for switching was because of how my use of email has changed. I no longer run a local SMTP server and no longer do local mail routing or fiddling with fetchmail, procmail etc. Main reason is that in this day of mobile devices, I tend to access my email from multiple devices (phone, tablet, desktop, laptop etc) and I want all my mail accessible on all devices. In basic terms, I now manage email via imap. At the time when I moved VM had some issues with IMAP, especailly with Google.
Another reason I moved was that I found VM's support for HTML mail wasn't great. Personally, I hate HTML mail and think it was possible the single worst thing that ever happened to email, but the reality is it is here and here to stay. I'm not sure what the current situation is with VM, but mu4e uses the recent EWW mode in emacs, which I find works really well for HTML messages.
One of the things I liked about VM was the ability to mange multiple 'folders' and to easily have my mail in different folders and search for messages. The search facilities of mu4e are powerful and fast. I am able to have multiple 'folders' for my mail, though there are some inherent issues with doing this if your a gmail users (see below).
The other thing I like about mu4e is that it has good integration with org-mode. I am a heavy org-mode user and the ability to both link to mail messages and use org formatting in messages is really useful.
There are some warts though. However, they are similar to the warts I found in many other solutions I tried. Note that I did use mutt for a while and I really like mutt. However, mutt from within Emacs just doesn't work and I really wanted to do my email in emacs. I also tried mew, which I think was quite good. However, there were some issues and it lacked the org integration I wanted. I also tried Gnus and Wonderlust, but never warmed to either of them.
Main issue I have with mu4e is that it doesn't actually do any mail retrieval from remote servers. You need to use some other program to do this. You can use fetchmail or similar. I use mbsync. It works well, but can be a little slow. However, if you set it up to run in daemon mode and check for new mail every x minutes, you don't really notice this.
I gind GMAIL to be the biggest problem and the one which took the longest to setup. The problem is, Gmail doesn't actually have distinct folders. It uses labels. All mail is actually just in one mailbox/folder. If your like me, this can create a challenging setup so that you get the right balance between having the mail you want and not having to wait minutes for everything to be downloaded. It also means that if you want separate local folders for the messages, you are downloading the messages multiple times. My way of working around this was to only have distinct local folders for the 'mailboxes' I visit a lot and then use mu4e's search facilities to present different views of the messages in the 'main' folder. This works quite well. You can setup key bindings for this. The downside is the time it can take to setup initially. However, once working, it works quite well.
Something I really loved about VM was pcrisis - I really liked how easy it was to change 'personalities' when sending mail. In mu4e, you have the ability to setup different personalities as well, so I can easily switch between personal, work, volunteer and other 'identities' I use.
All in all, while it has been necessary for me to establish a new workflow for managing my email, I find with mu4e that I'm as fast and productive as I use to be with VM. I have a much smaller 'inbox' and better records in org-mode and overall, have probably got a more unified solution than I did with VM. The eww browser mode in Emacs works really well and if absolutely necessary, I can open a message in an external browser with no difficulty. I don't use BBDB any more as I prefer to manage my contacts in the cloud so that they are available across devices. The ability for mu4e to use such cloud data is not great, but it does automatically maintain contacts from messages you have received (but to be honest, I haven't looked hard at this level of integration yet).
All in all, if your thinking of switching from VM (and I'm not suggesting you need to at this stage - if its working for you, stick with it as long as you can!), consider mu4e. Like all change, there will be a transition period and a degree of frustration, but in the end, I think much of what I liked in VM is satisfied with mu4e.
Of course, none of this detracts from the fact email is becoming less relevant. I note from speaking to my kids (both over 18) that they hardly use email at all and rarely even check their email accounts. Even my sone tells me that most of his interaction with the administration at the University he is at is over other channels, such as instant messages, twitter and various chat services. As far as they are concerned, email is this old fashioned thing from the past!