Short answer: yes.
Long answer: it depends what you want to do with it...
The first thing you NEED to do is download and install Opera Mini; you cannot get anywhere via the BlackBerry browser--Boost obviously does not offer a BlackBerry data plan, being that none of the "real" Boost phones are BlackBerries. (You will, however, want to keep the BlackBerry browser installed as it is a dependency for a lot of other good apps.)
Do NOT pay the 35 cents per day for "Wireless Web" use. This Boost "add-on" has no impact on whether or not you can get online with a "Boostberry." Slap a Boost Mobile starter SIM in your 7520 and you magically have UNLIMITED DATA for a dollar a day (or whatever plan you choose.) Yes, it IS awesome. The "free" data has something to do with the BlackBerry's native TCP/IP something or other blah blah blah... however the magic happens, you're bypassing Nextel's "Wireless Web" which is what the 35 cents per day on a "normal" Boost phone will get you.
I paid the 35 cents for the Wireless Web add-on one day as an experiment and noticed absolutely NO added functionality versus the bare-bones dollar-a-day plan.
Some people suggest paying the 35 cents per day so that Boost doesn't start "cracking down" on this loophole. But poor people (i.e., the kind of people who actually use Boost BlackBerries, i.e., me) recommend not paying it. The dollar a day plan plus 35 cents is going to be very close to $50... and very likely more than $50 if you use some daytime talk minutes here and there... if you really feel obligated to pay for the unlimited data, just go with the balls-out $50/month Unlimited Plan.
Anyway, the number of people running old Nextel BlackBerries on Boost is probably small enough that Sprint would never devote the resources needed to implement a way to stop us from getting our internet access.
Moving on... incoming text is working fine for me (7510) and my wife (7520). Others have apparently had different results in the past.
For SENDING texts, you'll have to reply through e-mail... there are a ton of "workaround" tutorials online for "texting" on a Boost BlackBerry, but in the end it always boils down to using a webmail client to send e-mails to an SMS gateway.
To make it perfectly clear: your 7520 BoostBerry will NOT send SMS or MMS... you can't hit "reply" from your inbox and shoot a text back like you would on a "regular" phone. I would recommend setting up a Gmail or MSN Live account specifically for "texting" use and then install the Gmail app or just set up a bookmark to the Gmail or Windows Live mobile site(s) in your Opera Mini.
To "reply" to texts via e-mail, you'll need to know the e-mail to SMS gateway suffix for your friends' network(s). For example, you can't send an e-mail to "5551231234"... because that's obviously not an e-mail address. Depending on what your friend's network is, you would have to send the e-mail to (for example) "firstname.lastname@example.org".
There is a list of e-mail suffixes by carrier available on Wikipedia if you search for the article "list of carriers providing email or web to sms."
Yes, it's a pain in the ***... if you want the Boost Mobile/7520 combo strictly because you think texting will be totally sweet with a BlackBerry keyboard and Boost's Unlimted Chat plan, you'll be very disappointed. But if you're more interested in being able to play on the internet all day every day for super cheap, the BoostBerry RULES.
If you can compromise and are willing to use mobile IM to stay in touch with your peoples instead of traditional SMS texting, there are IM apps that will work nicely on the 7520. I use Nimbuzz, which is a completely free one... it lets you simultaneously connect to your MSN, Yahoo, AIM, GoogleTalk, Facebook and, I think, ICQ and MySpace as well. (But I would not recommend adding your Facebook account because it seems to take about a friggin hour to load your contact list after you do, and then all the Facebook contacts show up as numbers instead of actual names.) Again, the unlimited data means you're not paying a single cent to anybody beyond your regular Boost plan, even if you IM for 23 hours a day.
Another cool app that runs smooth on the ol' 7520 is Google Maps mobile. The downside is that even though the 7520 has GPS, the Google Maps "My Location" feature doesn't seem to work on the platform. I have not had good luck with finding other free map/GPS apps. Even without GPS linkup, Google Maps mobile is great for taking turn-by-turn directions with you if you don't have a Nuvi or Tom-Tom or whatever.
The ringtones/wallpapers definitely suck a large quantity of ***. I don't know if that's a concern for you. Downloading new crap is next to impossible since you can't "push" (download) images or programs into your BoostBerry through Opera Mini. If you try, Opera just invokes the integrated BlackBerry browser, and you won't have a working service book for the BlackBerry Browser (which is apparently the only means of interfacing between the web and the BlackBerry file system.)
In short, ANYTHING you put on your Boost-enabled 7520 is going to have to be downloaded to your PC and then pushed via USB... and this only works for actual BlackBerry application packages or software updates... it's not as if you can use a BlackBerry with 32MB of internal storage and no memory card slot as a USB drive. So, in short, you can't even transfer a JPEG to the 7520 without "real" BlackBerry service, which is absolutely infuriating... I am at the point where I'm ready to break out my old Computer Science books from college and try to compile a Java "app" just to package up a few wallpapers and stick them into the BlackBerry file system. (Incidentally, if anyone has done this, I'd love the source code.)
Search this forum and Google... there are literally hundreds of pages of info out there dedicated to optimizing "BoostBerry" devices. A good percentage of them are right here on the CrackBerry forums if you spend some time searching. There are obvious ups and obvious downs... whether it will work for you depends 100% on your personal priorities and needs.
For me, money was the main motive... I got a 7510 and a 7520 for 15 bucks each and the 64k Boost Starter kits (include $5 initial account balance) for $5.99 each (all on eBay... I think prices tend to run a little higher than this but I got lucky with some sweet deals.) I spent less than 50 bucks altogether and got "new" phones for me and my wife along with 4-5 days of unlimited internet, mobile-to-mobile, nights/weekends, and incoming text. If nothing else, it beats the living **** out of the $30 "top of the line" Tracfone with no airtime included, absolutely terrible call and text rates even with the "double minutes for life" gimmick, and no hope of using the internet unless you want to throw down for a $60 airtime card approximately every other day.
Yes, the 7520 looks like a dwarfism-afflicted typewriter with a car antenna sticking out... couple that with a pay-as-you-go plan and you're basically looking at the ultimate stereotypical crack dealer phone. But for me, the ghetto chic of the BoostBerry is part of what makes it so appealing... (that, and the fact that I can't get approved for a "big kid" cell phone plan anymore haha.)
Hope that was useful... I'm still figuring things out myself, but feel free to let me know if you have any other questions.