Digitech's RP line are computer chip driven multi-effects floor units boasting effects like flanger, phaser, rotary speaker, tremolo, envelope filter, chorus, delay, and reverb along with amp simulations of popular amps. Some have an expression pedal for assigning an effect to like the RP300A.
The RP300A features 1/4" input and stereo output jacks, headphone output, jam-along 1/8" input, and it includes an AC adapter to power it.

Let me just say, the presets might impress you in the store, but when you get home and really start playing you will want to start creating your own presets, which is fairly easy. Don't be deceived by the advertising, you can only use one effect at a time and not 12, but fortunately the effects are seperate from the compressor, delay, and reverb; so for example you can dial in your compressor settings, then your amp model, your effect like chorus for instance, then your delay settings, and then finally your reverb. But using chorus and phaser at the same time; you can't do that.

The effects are fairly good, maybe not as realistic as if you had individual stompboxes from Boss or DigiTech for each effect, but they certainly aren't junk by any means. They are quite usable, and they have up to 4 parameters to edit for each effect, which can be tedious but gives you more control. If you want LESS paremeters to edit and more simplicity then maybe the RP200 or 150 is for you.
I played a handful of instruments through this, mostly a Fernandes LP Junior style guitar and a custom Telecaster. I dug the clean sounds of the Blackface model with my Tele, and the Rectified model for distortion. I soon became addicted to swirling delays and whooshing flanger and phaser effects, and the spacey vintage vibe of the rotary effect. I was on an effects high for weeks. The compressor definitely comes in handy for getting even sustain as long as you arent heavy handed with it.
Ultimately, the amp models can sound a bit synthetic, and the effects don't sound analog, they are fairly obviously digital, but I think running into a good tube amp would greatly improve the sound from what I was playing it through: my old solid state Peavey combo.

I would imagine the speaker cabinet models would help in making this sound better for going direct into a PA system, but I still wouldnt want to unless I had to. Some comment it becomes thin.

Hmmm.. what are the effects used. Wellllll... this preamp/effects unit was definitely "it" back in the day... and yep. I'm the original owner. This is based upon the regular, 1-S-disc,no Artist,no chrome, no nothing GSP 2101. This is an analog preamp wed to a 24 bit digital effects processor, as well as including analog e.q.'s,compression, etc. The range of effects was pretty wide.. pitch shifting,chorus,delay,many different reverb programs ranging in complexity from basic multi effects grade reverb to close to studio quality programs... (with a veritable plethora of parameters to adjust..enough to give Bill Gates a headache.)
Not the MOST user friendly unit... but something that should be looked at as almost an instrument on its own.. It is fairly intuitive if you are just looking to adjust factory programs on your own, but if you are looking to create your own algorithms... then grab some coffee. Arm yourself with patience, the thick manual and a highlighter pen.. and you will be rewarded! ( once you understand the creating,mixing and saving patches.. it is easy... and the flexibility is unreal.)
Connections are 1/4 inch and XLR (with/without cab simulation),MIDI, and effects loop to sidechain other stuff. 2 rack spaces... Dunno if this has any USB thingie... hmmm

Easy to moderate for basic editing..a bit more difficult for creating patches, and deep parameter manipulation. Gosh. That almost sounded dirty. The manual is clear...a bit heavy to the Digitech propaganda side...however it is thick, comprehensive and has a bit to remember. Not for the plug and play guys. I mean it.

Good to great... The phaser is unreal for trippy, ambient Robin Trower, Pink Floyd stuff... with a LOT of different parameters available. Pick your wave shape. Same goes for the Chorus. Lots of delays available... very clear... total control over milliseconds you want.( no tap tempo).
Pitch shifting, extensive reverb programs ( with control over early reflections,high, low, mid freq. rolloff... and that is just the BEGINNING).
Tremolo... super. Compression.. eh. E.q.'s.. well.. there are a LOT of them. Graphic, parametric,notch... it has 'em. Panners, mixers, flangers, etc. 20-20khz clarity.

Ok.. So. The effects are pretty yummy. Just have to invest some time to really get to know the unit. The preamps... welllllll... again good to great. The cleans... amazing. I dialed mine in next to an old Fender Twin. I think I got my rig to sound sweeter. Mid level gain stuff... very very good with delays, etc. I get some great blues, edgy rock sounds. Nice and crunchy. (no factory presets...you have to adjust them to your taste.) Heavy distortion.. here is where it becomes a matter of perspective. I would say...do not use this amp with V30's or G12-75's....I would say it does best with an organic set of speakers and a good tube power amp. It seems to become a bit sterilized with modern Celestion heavy rock type speakers, and benefits greatly from some of the Eminence stuff or Greenbacks. Something warmer. This unit is great for tight hi gain rhythms.. i have had a love/hate relationship with this unit for leadwork. It does not get a very organic tone for lead.. but I have only used it with 6L6 based poweramps... maybe with an EL34 setup it would be a bit warmer... Less tight. More note bloom. Currently, I just use this as an effects device for my Voodoo Labs preamp.. but did use it extensively as my primary tone generator for years... ( Yep. I have had this unit for about 15 years.. still chugging!!) Use it with Les Pauls, Strats, Tele's, metal type guitars.
It sort-of respects the differences between these guitars... but really... on the high gain stuff it has it's own personality that it globs on to whatever guitar you play..I kinda hate that. The good news is... you can play a 2k Les Paul.. or a 200 dollar Rogue... and get them to sound close, quality wise

Analog preamp + digital multifx in a 2U rack
Got more effects than you'll ever need in this unit

- you got to be patient when you setup the 2101, the effects chains can be very complicated: lots of paraméters, same effect can be used and re-used at any place in one chain. The manual is well designed and very helpfull

The effects are really good. 2 groups:

- analog (préamp): compressor (excellent), distorsions (transistor or tube, they both sound very good)
- digital (multifx): the wha is cheap but otherwise all the effects are great (you've got here all the effects you can think of, even the whammy). You can blend the ratio original signal/processed sound with the mixers in the chain. I only had problems with the réverbs: lot of people find it great, i don't (T.C. electronics reverbs sound 10 times better to me)

You can really achieve uncommun sounds ala allan holdsworth etc. Everything is possible!