I couldn't live without one, and the more I use it the more second nature it becomes. It saves a LOT of time setting up, I do a fair bit of on location "studio style". Not just lights on subject, backdrops, light spill, etc.
Using Speedlites which I'm generally shooting at f8 and more open. Speedlites aren't all that powerful, so are generally used close up. This means even small movements from a subject or height changes can make a big difference.
Mine is the Sekonic 308, got it for about £100, can't fault it. I tend to meter for portraits/flash, but for other work I've been using the camera's meter, but I feel I should use the meter more in these areas, too.
I came close to selling it last year, then I learnt what I'm doing and oh boy I am glad I didn't.
It's one thing thinking you have it right on a shoot, it's a whole lot better knowing.