Where are you putting the DHP-55 in you effects chain?
Right now, it's in the effects loop of my Marshall DSL 100. Well, _technically,_ the Marshall is in its effects loop due to the somewhat-strange-at-first-but-makes-sense-if-you-think-about-it nature of the DHP 55's separate pitch-recognition inputs. Basically, my JEM 777DY goes into a Boss SD-1 Super Overdrive and PH-2 Super Phaser and then directly into the right input of the DHP, and the "Effects Loop Out" goes into the input of the Marshall. The Marshall's Effects Send goes into the DHP's left input, and then the "mono/left" output of the DHP goes into the Marshall's effects return. Everything gets dumped into a Marshall 1960V with 4 Vintage 30s. Whew!
Outside of getting a good gain kick from the SD-1, I pretty much fly solo with the Marshall + the DHP. I'm planning on going rack one piece at a time, however, so I'm sure I'll wind up mating the DHP with various 1U goodies ...
I've also heard great things about the "String Swell" patches. Are these like the ones found in the 2120?
Not being familiar with the 2120's "string swell," I can't give you a good comparison. I can tell you that the "string swell" is seriously trick, though. It really works best with non-distorted sounds -- it's basically a chord shift of one octave that swells in and really gives you some outrageous sonic stuff to work with. Very very fun! Now you can ditch your keyboardist too! Ha ha.
Oh yeah, how's the arpeggiator?
There are actually two ways to pull off arpeggiation. The first, and most obvious, is the "four voice pitch shift with delay" algorithm, which runs stand-alone and is unintelligent (non-diatonic). That one works fine for special effects, and each regeneration of the delay is shifted as expected.
The second way to pull off arpeggiation is to run a diatonic/intelligent harmony, pan each of the outputs hard left or right, and pipe that into the stereo delay with separate L/R times for each channel. That gives you "one shot" diatonic arpeggiation at the expense of shifting the regenerations.
Any other wacky effects?
The box is kinda loaded with goodies. As I mentioned, you've got ADR envelopes and 2 LFOs to play with per patch. Something fun is a little patch I created called "The Bends," which uses the ADR envelope to bend the harmony up a minor 3rd. Dig: When you play at a "normal" level, you get a straight two-part diatonic harmony. However, when you strike a string hard enough to kick off the ADR envelope, the harmony starts as expected, suddenly bends up a minor 3rd, holds for .5 seconds, and then slowly (over 2 seconds) returns to pitch, all the while the original note from the guitar is playing. Gives you that Metalica "Blackened" intro feel (I will likely be sued by the band for even construing the patch to begin with, as they hold copyright on the minor 3rd bend).
You can also assign volume to the ADR envelope to give volume swell/backwards guitar effects. LFOs can modulate harmony vibrato, volume, panning, etc. It's not as open-ended as it could be (you can't modulate delay time with the LFO/ADR, for example), and you'll notice it's an ADR envelope and not an ADSR envelope (no sustain level parameter) ... but still, for a $200 piece of gear you simply can't go wrong.
I'm starting to get into the "chord shift" algorithm, which is pretty much a polyphonic pitch shift. It's somewhat restrictive (only shifts +1 octave) and there's a noticable volume drop when it's in use, but it yields some very cool tones.
Two other things worth mentioning: First, don't get tricked into thinking that 1994 DSP technology = old and moldy. While it's true that DSP has come a long long long way in the last 10 years, remember that diatonic pitch shift really fell out of favour in the early '90s when everyone started scrambling to make germanium-transistor fuzz boxes and retro kitch gear. No one even though to use DSPs again until the late '90s, and it was pretty much limited to "modelling" old gear. Outside of Eventide and the "vocalist" market, diatonic/intelligent harmony is a lost art. Thus, what you get in the DHP is pretty much "state of the art" even for today.
There really is some impressive processing power in this puppy: In one patch, you can have a noise gate, compressor, 2-part diatonic harmony, chorus/flange/filter (one of the three), 6-tap 3 second delay AND a 2-channel 1.5 second stereo delay at the same time. Throw in the two LFOs and the ADR envelope and some realtime MIDI stuff and you've got an entire rack in 1U.
Second: May I just say that the input/output knobs on the front panel are super-cheesy? They're "set and forget," but still ... those things feel dangerous!!
I've obviously rambled a bit ... hope this information is helpful!