I'm of the belief that photographs have a natural print size. Some photos are meant to be 4x6 and look silly blown up, and some don't feel like they can breathe unless they're 20x30". Sometimes a little print just doesn't cut it.
When I print in the studio I use an HP B9180, which I like a lot, but I'll write about that some other time. The problem is that the maximum print width is 13". With a 13x19" print being about the maximum you're going to get out of it. If I want to go bigger, I send them out.
A quick aside to those who don't think there pictures will look good at 20x30" because it won't be 300dpi like you're supposed to have... Don't worry, if it's a sharp photo with good color, it'll look great. Modern cameras, especially in the 10-12MP range up-res quite nicely. Plus you're not looking at a poster with a loupe anyway, you're standing a couple feet away usually.
Hell, billboards are only 10-20dpi. So taking the long dimension of my 5D 4368px/10dpi/12inchesperfoot = a perfectly nice looking 36FT billboard.
I've done up to 24x36" from a 5D file taken handheld with a 24-105L zoom lens and they look great.
Last year I entered some prints into a competition in London, and needed to get prints done fast, so I went onto Adorama's website, uploaded my final jpegs, and then went to pick up my prints the next day. Well they didn't have them done because there was a problem with their machine or something and they didn't know when it would be fixed. I was livid because they could have at least emailed the customers to give them some info about the problem.
I was flying out in a couple days (they had to be hand delivered) so waiting for Adorama wasn't an option. So I called a place in NJ called El-Co Color that I had used to make posters once or twice before and boy am I glad I did. It was 3PM and I asked if there was any chance they could print a few posters and get them out the door by the end of the day. The woman on the phone said, "Sure, just tell them Doris said it was ok". So I went online, made sure to tell them what Doris said in the 'special instructions' field, and uploaded 5 jpegs by 3:30. At 3:45 I got an email saying they were printed and sent to shipping. Yes, 15 MINUTES later. They arrived the next morning, UPS ground and they looked fantastic. Colors were spot on, just like my screen (calibrated Eizo) and printer.
As a funny coincidence, as I got back from my computer after signing for the El-Co package, I found an email from Adorama saying my prints were ready. Curious, I hopped on the F train and went to pick up my prints. The results? ...were awful. I expected two big labs like that with the latest gear would come out similarly, but boy was I wrong. The resolution was fine, but the colors were way off. If I didn't know my files were right I'd think I had done something wrong; but with my monitor, printer, and el-co all agreeing nearly perfect, the culprit had to be Adorama. I don't know if their machine was still busted or I got a bad tech on a bad day, or they have a far too heavy handed auto-correction circuit. Whatever the reason, I won't be going back there for my printing needs. And to be honest I don't like their recent store redesign at all either.
I'm sure there are other poster printers online that do good work too. I've had experience with one other place I can't remember the name of, Print-something... Anyway, the prints were ok, much better than adorama, worse than el-co. And they were large format inkjet prints where the el-co ones are traditional color prints on Fuji paper. Oh and the best part is the price. A 20x30" poster from El-co is, wait for it, $9.95. I know crazy, right?
And no I have no affiliation with them, I just thought their work was really great.