I think Meshtastic is great. I am not a Meshtastic expert, and (for better or worse) I deliberately tried to not learn anything about Meshtastic while building ChatterBox, so as not to unintentionally "borrow" ideas. I think that has led to using a fundamentally different approach to some of the same problems they have solved.

Here are some differences I have become aware of so far:

Self-Contained Device
There is no reliance on apps, phones, or any other external system. Everything involved with meshing, caching, repeating, is a capability that all ChatterBoxes support, right out of the box. This allows them to be quite capable as a group. There are not different types of devices you need to buy to get the different capabilities.

Smart Meshing
ChatterBoxes use live RF traffic monitoring and other trickery to gain an ever-adapting understanding of how devices within your cluster are connected, so they can efficiently push messages through the mesh to to recipients in the shortest path possible. They do not simply parrot messages in hopes the message will be lucky enough to get where it needs to go. I believe this is substantially different from meshtastic.

Mesh Cache
Across all ChatterBoxes in your cluster, a shared/distributed "mesh cache" provides a decentralized place for message packets to securely be stored, while they trickle out to where they need to go. Message packets can live in here for up to 24 hours while they wait for the target device to come within range or power on.

Asymmetric Encryption
Your messages are guaranteed to only be readable by the sender and recipient. If multiple mesh hops happen along the way, even other trusted devices that are part of your cluster cannot decrypt the message. They only know enough to cache the encrypted message for a while if necessary and help push the message along to where it needs to go. That is accomplished by encrypting using elliptic curve public/private keypair.

Digital Signatures
Every message sent through ChatterBox is digitally signed using ECDSA. If you receive a message from someone, you can be guaranteed it is from their device and has not been altered en route.

Signed Delivery Confirmation
If your message is successfully received, decrypted, etc, the recipient's device will send you back a signed acknowledgement. You will just see a checkmark or other icon next to the message you sent, but when you see that you can know for a fact the other device received your message. You will get this confirmation regardless of whether the recipient was in range of your device (so a direct send) or whether the target device was powered off for a while or out of your range but received your message via mesh.

More than just Text Payloads
ChatterBox is built atop a "Chatters" secure communication layer, which can handle text or binary data (such as images). ChatterBox is only supporting text on day one, but data is baked into the protocol, and we will build interesting features around that. All the same encryption and digital signatures apply, regardless of whether the data is text, images, or anything else.

No App or Invasive OS
There is no app that could get disabled or removed from an app store. There is no OS involved that can get a bad patch, spyware, or other risks that are present in other solutions. This is a deliberate design decision and made the development much more difficult, but also ensures there is no OS that gets a peek at your messages after the decryption is done.

DIY-Friendly
You don't need to know anything about complicated settings or radio frequencies. When you get a new device (with firmware installed), the device walks you through the one time cluster setup process. Other than what you want to name things and your time zone, you can basically stick with defaults and get a good setup. That first device is your cluster "root", basically the admin device. From there, adding new devices to your cluster is as simple as clicking "Onboard a Device" in the root, and clicking "Join Cluster" in the device you're adding. If your adding a new node, all you have to do is power on the node and then on the root device, choose "Onboard a Device". The onboarding itself happens over LoRa.

DIY-Friendly
I think Meshtastic and ChatterBox are probably similar in this way. I carefully chose components in such a way that a person can purchase parts from various sources (probably even entirely Amazon), and build their own ChatterBox from scratch with some basic soldering skills. This should lend it to be repairable as well. A yet-to-be released version of ChatterBox is almost entirely composed of Adafruit components.

The firmware will be made available very soon, and you will be able to install ChatterBox on compatible devices you can purchase from various suppliers.

Partially. The mesh graph visualization tools and cluster simulator are open source and can be found on GitHub.

The core of ChatterBox, including the encryption and mesh layer are not open source at this time.

More will be open sourced over time, keep an eye on the GitHub account for more repos becoming public.

No. The messaging is only between on-cluster (trusted) ChatterBoxes. It does not integrate with SMS, cell phones, or any other messaging systems or devices.

The encryption/digital signatures/mesh graph/mesh cache/etc all only exist for your cluster on ChatterBox devices. They were designed to be private and secure, so they only send encrypted/signed messages to other ChatterBoxes.

Yes. You should not have to replace batteries very often if you use good quality rechargeable batteries.

The difficulty or ease of changing batteries really depends on which device you have.

Whether or not two particular devices are in range with one another is controlled by many factors and it's really impossible to predict the range you're going to get in your area / building / city / etc. If you are in a concrete building and not near any windows, you're not going to have good range at all. But, once you walk of the building, your device may start sending/receiving packets and messages that were held while you were inside. Things that are good for range are..lots of open air between two devices, a device on a high location (on a hill), one or both devices having upgraded antennas. Things that lower range include buildings, walls, foliage. Hills between two devices essentially kill a signal. However, a node or other device that provides a path over or around the hill will fix that problem.

If you send a message and there are no devices in range, your message will be held on-device for up to 24 hours while waiting for you to come within range of another node or device, at which time your packets will be added to the cluster-wide mesh cache or instantly delivered, depending on the situation.