Here is my experience with Bulk SMS for those of you who are interested in Bulk SMS.
When I first started Bulk SMSing, I went with using GSM modems. I live in a country where previously, I could sign up for a plan that had unlimited SMS! So what I did was I purchased a number of GSM modems and then signed up a number of plans with multi sim cards and started firing away SMSes.
Here is the maths for those of you who are interested in the maths.
8 port GSM modem sends roughly about 5000-7000 SMS an hour. Number of plans I had to sign up for a 8 port GSM modem: 3!
So I was sending non stop from 9 am to about 8 pm everyday! So roughly I was averaging close to about 80,000 a day with a 8 port GSM modem.
I was sending for all kinds of businesses from real estate to websites to car insurance etc. In fact, I was averaging about 3 Million SMS per month.
I was banking for sure. But after that the Telcos stepped in and told me I could not use my accounts to send so much SMS. Yeah, I know. I was sending huge amounts.

So I stopped it.
Anyway, am now back into the mobile advertising after about 2 years absence from that scene. And have recently moved up market to become a SMS aggregator using gateways.
Hence my observations are:
PROS and CONS of GSM modems: Good starter for newbies and if you are still able to find a provider who offers unlimited sms plans. Go for this route. You will bank money for sure! Speed is not very desirable. One GSM modem will roughly send about 500-700 msg an hour. While a bank of lets say 8 modems will send about 500-700x8 an hour. Cost effective. But you will need to be on a post paid plan. If you are using prepaid or top up cards, it will definitely affect your performance a bit.
PROS & CONS of Direct SMS: Speed is a lot faster. You can roughly pump out about 30k SMS an hour and if you have 3-5 servers, you can pump out a lot of SMS. Can be customized. Price wise is going to be slightly more expensive than GSM modems, but if you are sending for your own usage, it definitely can work out to less than what you pay using SIM cards. But because SMS aggregators need to earn a living and pay those hefty rentals and workers fees, it definitely looks more expensive in the beginning but I think it costs the same if you were using a top card up.
Hope this helps.