TL;DR: Picked up an SCR-DI yesterday (in large part based on the reviews in this thread) and agree whole-heartedly with those who love it. It has knocked a BDDI off my board (quite easily).
So I've tried a bunch of preamps over the years and have always come back to my old standby, a BDDI. I recently purchased a TC Electronics BH250 to use in my practice room and I just haven't been thrilled with the the way the BDDI interacts with it. Maybe after a dozen years or so of BDDI, I'm just sick of it...who knows. So yesterday I went down to Sam Ash on 34th to see what Larry Hartke had in stock in the way of preamps. I had no real ideas for what I wanted, mostly I just wanted to see what was there and see if anything grabbed my attention.
Once there, Larry showed me what he had. A Hartke Bass Attack 2 (of course...lol), an MXR M81, an MXR M-80, a Darkglass B7K, a BDDI, an Ampeg SCR-DI and a few others that are escaping me right now. I've owned (and sold) all of those with the exception of the Hartke Bass Attack 2 and the Ampeg. The Hartke is $50 cheaper but cost wasn't going to be the deciding factor. I had the first Hartke Bass Attack pedal and wasn't all that happy with it, although this newer version seemed to be an improvement. Be that as it may, I quickly decided I would be walking out with the SCR-DI or nothing at all. Knowing next to nothing about the pedal, I pulled out my phone and did a quick search on Talkbass which landed me on this thread. So I sat there in the Hartke Bass Lounge and read through this thread and have to say, you guys sold me on it.
Once I got it home and plugged it into the BH250 it took all of about 30 seconds for me to know I finally found the pedal to knock the BDDI off my board. The descriptions others have used in this thread (more natural organic tone, Ampegy, etc.) were all quite accurate. I set the EQ controls on the BH250 flat and was immediately able to dial in a very usable, rich Ampegy tone with the SCR-DI. Its hard to imagine an easier pedal to use in terms of dialing in tone. I also found the Scrambler to be quite good. Again, it took almost no time to dial in a very usable distortion. As others have said, I think you'd have to really work to dial in something "un-usable" with this pedal. After I finished A/Bing with with the BDDI, I added it to my board and it plays well with my other pedals (Source Audio Bass Envelope Filter, MXR Bass Distortion, TC Electronics SpectraComp, and my non-tone shaping pedals: a BeatBuddy Mini, TC Electronics Ditto and TC Electronics WireTap).
I have yet to use the Aux In/phones feature as the BH250 has the same feature with the added bonus of piping the Aux In *and* my bass signal through to my cab so I can practice using the Aux In with or without headphones. But it will be nice to have the ability to use the SCR-DI as a stand-alone practice tool for times I don't have my BH250 handy (although because the BH250 is so small and light, I almost always throw it in my gig bag anyway). It never hurts to have redundant practice capabilities.
Bottom line - the SCR-DI far exceeded my expectations and has definitely found a home as my main tone-shaping pedal. Thanks to everyone who contributed to this thread - tons of useful posts that really helped me get a sense of the thing before I purchased it.