I used to use a couple of Streak's features via standalone plugins for Chrome, but none of them did a good job anyway. Streak combines things like email snoozing, snippets, scheduled sending, read receipts, plus an entire pipeline CRM to Gmail all with one simple plugin.
As a small business we simply add emails from our website's Contact Form to a Streak Pipeline and we can make notes, and see all emails and contacts involved. The data has been helpful for us, too. For example, how many leads convert, and what's in the Notes of the ones that don't? Very valuable info for us!
Since Streak is a Chrome plugin it can sometimes alter the page in a way that appears broken, or breaks other Gmail features. These bugs are tiny, and they come/go with new versions being released. I think this is just a limitation of it being a plugin, though.
The 'Split Thread' feature has never worked for me and I think that'd be a handy one. I'd like to see thumbnails in the 'Contents' section for attachments, or just a totally different section / label for a Box's attachments.
If you deal with customers primarily by email this is a great tool to keep track of threads as they progress from leads to sales.
I used the scheduled sending to make it look like I'm sending emails bright and early in the morning, and then my message is at the top of the receipients inbox in the AM. I use the Snippets feature to copy in information that I found myself retyping to clients during our order process and saving TONS of time. If someone tells me they will follow up, I use the Snooze feature to pop the message back into my inbox if no one has replied. The CRM feature is great, as well. I use it to follow email chains through our order process and the Boxes are helpful for clients who start new email chains on every reply, send attachments separately, multiple chains per order etc.