TL;DR: It's a huge upgrade over Lightning. It has higher FPS, much better battery life, and much snappier trigger response.
I have got my vesper for few days, and have used it in CQB games, here is my thoughts about this blaster:
First of all, what I have bought along with this blaster: CNC trigger, smart display riser, and official bcar barrel (not GFZ one). The smart display riser is a must for me, as it can fine tweak many settings for this blaster.
FPS
This blaster is said to be 180FPS, but for some reason, I get an averaged 165 FPS with worker 1g fresh darts, while Lightning has around 135-140. And with used darts, vesper can still achieve 13x FPS most of the time, while lightning's drop to around 11x FPS. That 20-30 FPS boost is critical in actual games, resulting noticeably longer effective range.
Trigger Response
Out of the box, it already has much better response time than Lightning. In my slow-mo testing, it has around 108ms response time, compared to lightning's 158ms. In the smart riser, two params can be adjust for the response time: The hyper speed and hyper wait, which are essential the idle pre-spin speed and the delay time after trigger pulled. With those set to fastest response, the time can be as fast as 58ms, but the pre-spin is much louder. I settled in the middle ground now with around 75ms.
Trigger response testing video
Accuracy
It's surely better than stock lightning in terms of accuracy, but it's still a bit disappointing for a blaster with this price point. With the official BCAR installed, the accuracy is improved with fresh darts, but with used darts I can barely notice any differences.
To address the accuracy issue with used darts, I made a 14.2mm bore barrel, hoping to improve accuracy with used darts, as it would be the most realistic use case. And I have successful result with it, now it can hit a fist-sized target from 20feet with 70-80% of shots using used darts, which is a vast improvement compare to stock barrel / BCAR barrel.
Barrel file available on printables
Battery life
The battery survived my 4 hours game night with 25% left, which was around 8 games. With lightning as a primary, I usually have to swap battery after 3 games.
Magazine support
As I have 4 lightning mags and 4 lightningale mags already, it would be great if they can be used with Vesper. They were tested to be compatible with this blaster by people so I didn't get extra vesper mags.
However, in my testings, they didn't work that well. The lightningale mags don't catch naturally, it requires some wiggling after inserting for them to catch. And the dart position is slightly higher, I had several times of feeding isssue happened at the game night, not sure if it's the reason or just the dart being too worn. Lightning mags catch naturally, but they still wobble.
The wobbling fit of mags did cause some problems in games. There was several times that the mag dropped when firing. As I am using full auto with short burst mainly, I suspect the fast and repetitive movement of the pusher crush the slightly misaligned mag and made them drop. And the vigorous movement in games may contribute to that too.
In short, I wouldn't recommend using lightning/lightningale mags for Vesper.