I have eplus based SIM cards for private use for several years and i have never experienced any dropped calls. The voice quality is very good.
For data o2 might be the better solution, but as my phone only supports GPRS there is no difference for me either..

In our company we have more than 500 simyo cards for data traffic (m2m applications) and the connection is always reliable (but our case scenario does not depend on speed, only on reliable connection and small data amounts).

Chris

@kuba.g
Fonic SIMs can not be recharged by credit card, but only by topup vouchers or a German bank account.

@Kritiker
A disadvantage of solomo pro is, that the SIM is only available online and can be shipped only to Germany and few neighbouring European countries.

@dg7feq
I don't know where you or your company have been using the eplus network, but three of my friends recently switched operators because they had continuing enormous problems with data connections as well as with voice calls with epus. Two of them live in Stuttgart and one in Hamburg - they reported not being able to establish data connections and not being able to receive or place calls while their phones were registered on the 3G-network almost every day during the rush hours.
When I was an eplus customer from 2006-2008 the 3G network in my home town was so congested, that with a probability of 80% incoming phone calls would not reach me, allthough I had very good signal strength.
Problems may lessen if you switch your phone in 2G-only mode, but all in all their network still is a pain.

Thank you all for your advice so far. It has helped me tremendously.

Since I too have had problems with E-Plus (in 2007) I am inclined to try Fonic. What's holding me back is Fonic's much higher roaming rate for calls from some country via Germany to some other country. Of course, if you can't make or receive the call, then it is cheaper still, isn't it? ;) The roaming does represent only about 20% of my intended use.

I'll be out-of-touch for a few days and will respond to the most recent, and any future posts, when I return. Thanks again.

Solomo surely have cheap roaming rates especially for outgoing calls through their callback service. However their callback service is unreliable and some phones (e.g. some Samsung phones) even don't support it at all, since it is a tweaked STK/USSD-solution which seems not to be complying fully with standards.
Further there are dozens of reports on the solomo customer forum of solomo SIMs not even registering on foreign networks.
You should know that solomo is based on a MVNO called vistream and vistream is a real MVNO running some essential core network infrastructure by their own while they basicly only use the cell towers of eplus. However vistream have a lot of problems with their infrastructure - e.g. a lot of users experience no calls and SMS coming in while there's an active data session.
This combination of the messy, overloaded eplus network plus the buggy vistream infrastructure results in an inferior product, which I just cannot recommend. You may end up spending way more money if solomo once again does not work.

Is it still true that to purchase a Fonic card online, you need to have a German bank account? I couldn't seem to get past that requirement when I tried recently. I will have a German Address, but no bank account during my visit this summer.

That question has been answered all too frequent. Use the search function or read the detailed article on Fonic linked in this thread.

Yes they can. I recharged mine online by credit card :) although not via Fonic.de, obviously ;)

I know of voucher resellers like aufladen.de, who accept credit cards, but afaik they do only accept Dutch, German, Swiss and Austrian credit cards. So these resellers are of little use for our American fellows.

Cabox, where I have recharged my Fonic once, also accepts PayPal :)

Inquisitor, I have indeed searched for this information, and would not be asking had it been answered satisfactorily. Most threads are from 2009 and before, and some imply that the requirement has changed in the meantime. The page you mention I've already looked at in detail, and it does NOT answer my question:

This addresses activation, not online SIM purchase. Hence, my question.