I'm going to go against the grain here with my recommendation(s), which are based on budget priority.
I also wanted to try out the NDxxx for the long exposures, but no way in hell was I going to shell out $100 or more to try it out. I found a used Hoya ND8 on ebay for $12, and I like it, but want darker still.
Last week I stopped my a welding shop, picked up a 10 Shade welding lens ($3.60), and made a filter. Came out ok, and it does give you crazy long exposures. I only took a couple of shots with it so far, and mostly just tests. With the welding glass, you do have to correct the WB, as they give a really dark green tint to everything. (See pics)
My next pruchase will be one of those really cheap variable ND flters you see on ebay for about $10. Yeah, yeah, they're cheap, and most of these guys will tell you they're garbage, and that's what I thought. But another forum member bought one, and posted a thread about them with pics, and while they aren't perfect, they are perfectly acceptable for a hobbyist/enthusiast. I can't find the thread, but it's less then a month old.
So, depending on where you are budget-wise, and whether you consider yourself a pro, amateur, enthusiast, hobbyist, or a guy with a cam, you have choices that run the full spectrum (so to speak). Once my cheap-o variND arrives, I'll take some shots and post them.
#1 - Untouched (resized only)

#2 - Quick correcting (about 2 minutes)