I've used MHelpDesk, and it does a very good job. It wasn't quite doing what I needed it to in relation to QuickBooks, though. For example, if I needed to sell a new product I had to create the product in QB, then sync it before I could use it in MHD. If I created it in MHD it would throw an error when trying to sync. This may be fixed now, but it drove me mad. The calendar in MHD rocks, and it does a very good job of tracking tickets, making estimates and invoices. If you're looking for a basic, affordable approach to running your IT business I think MHD will suffice.
MHD really gave me the stepping stone I needed to start getting involved with service tickets and such. Prior to MHD I was doing everything in QB and my Google Calendar. I've recently taken "then next step," though. In Feb I purchased a server that I'm hosting N-Central on, which has been working absolutely wonderful. In March I subscribed to ConnectWise with their Procurement module for tracking purchase orders and inventory. This has been a truly amazing and robust pairing. My RMM alerts trigger tickets in CW, allowing me to easily stay up on everything from one place. All of the devices that I monitor are also in CW including serial numbers, etc. Service tickets can be tracked against them, automatic events can be triggered on warranty expiration, etc, etc. I track all my leads, purchases, estimates, invoices, clients, etc though CW. CW also does a very good job talking to QB. That was the fallback I had with MHD - it did a great job at what it did, but I found myself still needed to use QB for a lot, which then had to be synced. It got a little cumbersome. With CW and N-Central, I can run my entire business outside of QB, then push any relevant information into QB for taxes, paying bills, and doing the normal "bookkeeping" of the business.
CW is insanely customizatable and is capable of doing way more than my little one man show needs. That can be one of its downfalls, too... It may be a little much for just doing the basics, but if you are looking for something that will allow you to run your business I think it's more capable than MHD at this point, especially if you do any sort of purchasing. The downside is it's definitely more expensive than MHD. Per license it's not much more than MHD, but they have a 2 licence minimum. This worked for me because my parents help answer phones, so they can create service tickets and such. Also, the procurement module for inventory and purchasing is an addon. All said I'm spending just over $200/mo for it. Well worth it to me since my business is ready for that "next step." Combining the PSA aspect of CW with the RMM aspect of N-Central, I have completely revolutionized my business this year. If you're ready for that step take a look at that pair. If you're just looking to maintain your clients, provide estimates and invoices, track service tickets and don't do much purchasing or inventory stick with MHD (or something similar) for now. I'm definitely not trying to discount MHD in the least, it's phenomenal. I liked it a lot better than ZenDesk and the like, so if that's all you need it for it's really the best option that I've found.
Hope that helps!