I have taken Colonia Express, and I was pleasantly surprised by it.
The terminal is small (when I went, the terminal was being renovated, and what was finished was lovely and modern) and there are much smaller crowds, and fewer lines.
It does the job of transporting you to your destination just fine, on a smaller, less luxurious boat. You arrive in the same terminal at Colonia as you do when you take Buquebus, so there is no downside on that end.
This is not the boat to take if you want the luxury ride with champagne and oversized leather seats, or if you want to buy a big meal and do a ton of duty free shopping. If you want to spend minimal money on a visa run, this is the boat for you!
Overall, for the price that I paid last time I used them (last April, I think and like AR$75 R/T to Montevideo during a sale) you can't beat it! They were easily half the price of Buquebus, if not an even better bargain.
Getting there - I disagree with the other opinions. You don't need to take a taxi. Take the 130 bus!! The 130 runs straight down Libertador, (which turns into Alem, which turns into Paseo Colon), and then turns on Av. Brasil, and then turns onto Don Pedro de Mendoza. The last stop on the bus is across the street from the Colonia Express terminal. The bus flies between Casa Rosada and the last stop (Colonia Express terminal). The taxi will drop you in the exact same spot as the bus does, and will cost you a whole lot more.
There is no danger in getting off the bus and walking across the street. The novio is from that neighborhood, so I've been taking the 130 bus to that stop every Sunday to visit the family for a few years now, and once six months or so on an early weekday morning to take the ferry. I've never had a problem on the 130! (or at Colonia doing a visa run, but that's another thread

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They removed the homeless people that were building a villa underneath the autopista, so as of last Sunday, the road leading to the Colonia Express looks much better and less dodgy.