It’s working out pretty well for me I’d say, and you can certainly build out a eurorack around an ES-9 as the beach head. It can do a lot, send any signal (CV, gate, audio) from the computer to the euro and any signal from the euro to the computer. There’s plenty of in/outs and the internal mixer and router is crazy flexible. It also serves as a regular output module of course, with stereo jacks and headphones out, and I have already used that for taking it out of the house and jamming a bit with some friends.
It is big, hot and expensive of course, so I guess the only question is whether the modules you need to do what you want, if you don’t have the ES-9, will take even more space and money. It all depends on your use case. It’s certainly well fitted for the scenario you describe, but if you already have an audio interface with ADAT, it might be worth checking out some of the other Expert Sleepers modules that only send signals from computer to eurorack. But if you already now know, that at some point you’ll be sending several tracks of audio and perhaps other signals, from the euro to be mixed and recorded in the computer, you might as well go for the ES-9 straight away. The more you are planning to use a computer with your eurorack, the more the ES-9 makes sense.
I have no idea what sounds you like, and what modules for euro you’re planning on getting, but just throwing out an idea - you could get a great start in the euro, with an ES-9, a Plaits, an Instruo TS-L and a Ripples 2020, as an example. It’ll give you hands on with a couple of great voices in the euro and all the rest - envelopes, sequencers, modulations, effects, mixing, recording, etc. you can use the DAW/Rack for. If you’re mostly into analog you can also check out e.g. the Dreadbox Chromatic line of modules, that seems to offer a lot of functionality at a very attractive price point, and I think they sound good. But there’s many ways to skin that cat For starting, a semi-modular in the euro is always great value for money, and I was very close to getting a Pittsburgh SV-1 as the backbone of my first system, love that thing, and there’s also things like the Behringer Neutron, the Mother-32 and others of course. But if you know that a couple of years down the road you’ll have a decked out 6U eurorack on your desk, you might as well start with discrete modules IMHO.
I would say the ES-9 is a great piece of kit and it hasn’t let me down yet.