reverb

Any suggestions on compatible units (pedals,etc..) for reverb (not echo)

One word.....

"LEXICON"

Don't waste your money on a lesser reverb unit, you will not be happy with the sound.

I have a decent echo pedal (Line 6 Echo Park).
I approximate reverb with my echo pedal. Is reverb really worth the investment as a separate fx ??

Everybody is going to have their own opinions so I'd say just try out several diferent models of multi-effects and make your decision that way.

I use an Art SGX=2000 and it sounds great.

Reverb and echo are so so so completely different it's not funny. Yeah, the echo park (I have one too - do you have crap noise thru it if you don't use a separate power supply?) can approximate reverb, much like a flanger can approximate chorus, but doesn't sound as good.

Alesis do some good units. The old Midiverb and Quadraverb units are still OK, the Nanoverb is a tiny cheap unit that would work well for theremin use, and still sounds good.
Zoom make a couple of minter rack units (RFX series) - I have the RFX3000 and it's reverbs are nice.
Some old Korg/Ibanez/Yamaha/etc units sound really good, some sound really bad. I have an old Yamaha R1000 reverb that's a little brash but the parametric EQ on it is killer.
Lexicon is, well, the lexicon of reverb. $$$ though, although they do make lower-spec units that still sound great.
Roland's old(ish) SRV3000 (space reverb) and the SDR3000 (space delay) are packed with other-worldly reverb, delay and modulation sounds.

Don't bother with a pedal version of reverb (Boss Reverb/Delay pedal for example), they don't cut the mustard. Apart from Danelectro's Reel Spring, that's not too shabby.

If you're DIY-inclined an old spring reverb tank (an Accutronics or similar) can be wired up with a microphone (electret or possibly piezo). Spring reverb is the most lush and natural I reckon. There's a company in the US that makes Accutronic-based spring reverb pedals, can't remember the name though.